Technically it should be called a heat exchanger, we lovingly call it the water cooler, but it has been providing us with free cold air for more than 20 years now so it definitely does work.
Shortly after moving here (Montana) I noticed that the water from our well is really cold, under 50 degrees. I built this heat exchanger to take advantage of that cold source for use in the house in the summer and as a byproduct it heats up the water going to the garden a bit before it goes on the plants since the plants didn't seem to care to much for the super cold water. Good benefits all the way around and since I would be pumping the water to water outside anyway the only actual cost is the power to run the box fan that moves the air through the copper piping.
How well does it work? We hit a high temperature in July of 112F, hottest that I can remember. The temperature inside was 76F with the cooler running all the time. I almost didn't want to go out to move the sprinkler.
The disadvantages? Well, you have to move the sprinkler a lot, but it does keep the grass green. Also if the humidity gets high, water will condense on the exchanger the same as on a glass of cold water. So I keep towels underneath it to soak up the moisture. If it gets really muggy I have to change the towels several times a day. I just hang the wet set outside in the heat to dry and rotate the dry ones back under the cooler.
It will take some skill to put it together but once done it is maintenance free, except for the time I left it outside before putting it away for the winter and it froze some leftover water in a pipe and broke it. Make sure to get the water out of it and store it where its above freezing.
Another disadvantage, it isn't pretty, but it is unique and truly "green".

Step 1: Check Your Water Temp

For this to work at all you need to have cold incoming water. The colder the better. Get a standard waterproof thermometer and run your outside water for a while and find out what temp it is. This probably will not work in places where you have a municipal water supply unless its from a cold source. Also you need to have enough ground to water so you don't water log your garden and lawn. It will probably work best in rural areas.
With the fan turned off and the water running outside the temp of my copper pipe is 48F (Infrared non contact thermometer reading)

Step 2: Materials

For my heat exchanger I used copper pipe with aluminum fins. This is the same stuff that is used for hot water baseboard heating systems. I got mine a long time ago so I don't know if the same stuff is still around but there should be something similar on the market.
In addition I got mine for free. I found it in a pile at the local landfill apparently discarded by a contractor as scraps from a heating job. (I used to get all my firewood at the landfill from trees people in town cut down, that has all since changed with all the new laws. Now I am not allowed to remove wood from the landfill, instead they bury it, go figure.)
I doubt that they throw anything like this away today with recycle prices so high but you should check with a local contractor to see if he has anything. He probably would rather sell it to you cheap instead of hassling with recycling it. If you can't get scraps then you will need to by new.
To fit a standard box fan the sections need to be about 21 inches long. Stacked one on top of the other it took 7 pieces to reach the top of the fan. This will vary if your fins are smaller.
You will also need elbows to connect the sections together and lots of little pipe pieces. Scrap works great here too rather than cutting up a full length of pipe.
Originally I started with one layer of pipe but after seeing how good it worked I added 2 more so mine has 3 layers of heat fins. This seams to be a good number. Anymore and it would get difficult to move around and would take up to much room. Any less and the air doesn't get enough contact to cool down enough. Finish the ends with fittings that can connect to a hose. You can use the special connecting hoses for washing machines but I just cut up sections from a regular garden hose. The bigger the diameter of the hose the better, you will restrict the water flow to the outside if you use small hoses and pipes. All of this hardware is available at any local hardware store. Shop around for the best prices. I used 42 elbows to put mine together.

Step 3: Tools

Not much is need in the way of tools. A pipe cutter is essential, Measuring tape and marker, Flux and solid core solder and rags to wipe off the pipes. The pliers are for holding the pipe as you solder, the sections are short so the whole thing gets really hot. You should also have some fine sandpaper or emery cloth to sand the fittings. It helps the solder to hold better. If you haven't done soldering before this will be great practice. Remember, do it outside or in a garage with lots of ventilation. Give yourself lots of space and watch for anything combustible. If you drop the torch make sure its not going to set anything on fire. Simple things for sure but people actually do manage to set things on fire a lot. Make sure the solder fills the joints completely. If you take your time and do it right you will have no leaks when you finish. Remember, hot liquid metal runs downhill and does drip. Don't hold it in your lap.
Assemble the sections separately then solder then sections together after they are finished. Make sure that the connectors for the hose are on the bottom, if you use 3 layers you will have to run a pipe from top to bottom to get both connectors on the bottom. You can see it in the picture of mine. One nice thing about using copper, if it doesn't fit right you can take apart the joints and do them over again.

Step 4: Reinforce It (tie It Together)

The soldered joints are not strong enough by themselves to hold this all together, especially after it is filled with water. I used plastic twine to tie mine all together because at the time that is what I had and it worked good. Today I would probably use nylon wire ties along with the twine. Just zip it all together. Whatever you use make sure that its tightly pulled together so the joints aren't stressed. It helps too to add some kind of handle so you can pick it up easily.
Notice in the picture the hose connector on the bottom and the long pipe to change from the top to the bottom.
One step I forgot to mention, PRESSURE TEST IT for leaks outside before you bring it into the house. Run water through it until all the air is out and then leave it sit under pressure and check for any wet spots.

Step 5: Connect to the Water Supply

I added a feeder pipe coming off my pressure tank and piped it under the house directly to a connecting box in the floor. I added valves, one on the incoming and one on the outgoing and a bypass valve. Then continued the feed pipe to the outside. Yes, if someone were to turn the valve on without the cooler attached it would spray water all over. But nobody ever has so far. The only time I ever had a leak was when I tried using snap on hose connectors. They are convenient but they leak. So they work great outside but don't use them for this application.
I should have used ball valves for the connectors, they restrict the water flow less and are faster to turn on and off. I may swap out the ones I have now to ball valves in the future.

Step 6: Turn on the Water

Place towels under the exchanger, check the hoses, turn on the water. You can hear the gurgling as the air moves through the pipes. Turn the sprinkler on outside. Turn the fan on behind the exchanger and thats pretty much it. Free Cold air.
Depending on what you can find, the initial parts could cost you a bit, especially since copper has gone up in price. And it is a bit labor intensive to put it all together. But once done its maintenance free unless it springs a leak.
And its totally green, using cold that would normally just go to wast.
Ours works so good that it sometimes gets too cold and we have to open a window. Other times when I need to water but don't need the cooling I have to put it on bypass.
I have to admit that I did get an air conditioner though. Its at the opposite end of the house and I run it when the humidity gets really high. Its mostly just for dehumidifying. The water cooler does most of the cooling.

Step 7: Answers to Questions

Due to the number of comments I thought it best to add an extra page to address some of them and to answer some of the questions and clarify a few things.

In regards to using car radiators - although at first this appears to be a good idea I would not recommend it for many reasons, some are as follows:
First - you would find it very difficult to connect up the water lines in a way that would hold the pressure. You would need to find adapters to somehow go from radiator size hoses down to regular water pipes. Even though it might be possible to do so, it is highly likely that you would end up with leaks.
Second - High water pressure would probably cause leaks. Car cooling systems are designed for relatively low PSI. Radiator caps are designed to open and vent at about 12 to 15 PSI if I remember correctly. Your normal house water supply runs between 30 and 70 PSI. My pump is set to cycle between 30 and 50 PSI (30 on, 50 off). This kind of pressure would likely rupture a radiator or cause the cap to pop open and vent. So, while you are outside moving the sprinkler and you turn off the water, inside a fountain erupts in the living room. Not so fun.
That's why I used standard water pipes - they are designed specifically to hold the higher pressure.
Third - Many older radiators leak to begin with. In order to keep them in service, many are plugged up with stop leak. But as soon as you start running lots of fresh water through them all of the stop leak and other assorted crud will be flushed out and they will probably end up leaking. And there goes your fountain again.
Fourth - Radiators are going to be difficult to get clean. Most of them are full of bug leftovers and oil and other assorted things from the engine compartment. Even if you do get them clean they will probably continue to put off a nasty smell.
Fifth - The ability to use the outside water in a normal way, and by this I mean that you can turn off the water at the hose, run sprinklers or even wash your car without any fear of fountains in the living room, is something you probably could not achieve using radiators. With my cooler there is no need to just let the hose drain on the ground because it can be pressurized - it is not just a drain. In addition because it is made to hold the water pressure and not leak you can locate the cooler anywhere in the house where you need the cooling. I actually considered building a unit to fit into the heat ducts and use the furnace fan to drive the air through it so that it would become a whole house unit, but I decided that it was too much work and too complicated for my needs. Keep it simple and you will have fewer problems.
If you are planning on using this for your garage or shop I could see using radiators. Water spills wouldn't be near the problem that they would be inside your house but I would not recommend using radiators for in-house use.
Next - If you mount one in a window and pull in air from outside you will defeat part of the cooling effect. The outside air is far warmer and has much more heat in it than the inside air. By setting this up completely inside and circulating the air inside through it you get more cooling because the air going through it is already more moderate. Use outside air only if you need the ventilation.
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What would be really great is for some enterprising company to manufacture a specific unit for this purpose. A heat exchanger/radiator that is designed for the standard water pressure built into a plastic housing for catching the condensation and with a fan mounted inside. An all in one unit. Just attach the hoses and plug the fan in. But it's unlikely that anyone will because the market is not large enough. Of course, that being said, some Chinese company will probably market this in the future and totally screw me out of any share of the profits.
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I did a Google search on the internet and there is baseboard heating pipe available, even some for salvage prices. Somebody in Texas had 200 feet that he had just taken out and didn't want to throw away because it was in ÃÂlike new shape. My cooler used about 40 feet of the pipe. So that's around the amount that you will need to make a similar unit.
The water I use is directly from my pressure tank. It's untreated and is intended for outside watering. Our ground water here is pretty nasty. It's very alkali and has lots of rust in it. Just to be able to use it for the house I run it through a sediment filter, then a rust filter and finally a water softener which uses rust removing salt. And after all of that we still don't drink it. So the inside water and outside water are kept completely separate.
I want to emphasize this---- I don't waste the water by just dumping it on the ground. It is used for watering the garden, grass and trees. It hasn't rained here for 8 weeks now. With temperatures in the 80's and above everything dies if it's not watered. So the water is not being wasted. I try to keep a band of green around the house as a fire preventative. In 1999 we had a grass fire / fire storm sweep through the area and 3 neighbors houses burnt to the ground along with 4 out buildings/garages and the UPS shipping depot. Only the houses with green grass around them were spared. So having a green belt is a pretty good idea. So again I emphasize, I would be watering anyway, I am just taking advantage of something that would normally be thrown away. (The cooling properties of the water). Also this doesn't need tremendous amounts of water to work. Even at a trickle there is still a lot of heat absorbed by the water.
"bricko" described this as Âa poor mans water source heat pump". He is pretty correct in that. Regular heat pumps don't work here because the winter air temperatures are too low, as much as 40 bellow zero and often 10 below for weeks at a time. So they developed what are called ground source heat pumps that both heat and cool using the ground for the moderate temperatures that a heat pump needs. My cooler is actually only half a system in that it's not for heating, but for cooling only. A full Ground Source Heat Pump system is very expensive and uses a lot of power year round. In addition the laws concerning them have changed recently. In Montana you are no longer allowed to return water back to the water table by using a well. There were too many instances of people contaminating the water table by returning dirty water to a well. So any new heat pump system has to be contained or sealed. You can run a fluid through underground heat exchangers but the inside fluid cannot come into direct contact with the ground water, thus it's a sealed loop system. I have avoided all of this by simply using the water for both watering plants and cooling the house. It's not sophisticated, there's no compressor, no thermostat, it's manually regulated, this is what makes it cheap and green. This is also in keeping with the whole concept of the going green contest. Yes there are more sophisticated systems available. But that's not the intent of the contest, at least as I understand it. (Or so I thought until I saw the contest results)
I do have a plan for a wind powered/compressed air/water pump that I would love to adapt to this system and make a totally self contained system. But I have never had the funds to develop it or the kind of shop tools I would need to build it. Maybe someday----.
Finally for those of you who have asked, I run the coldest water through the outermost layer so the air to leave hits the coldest water as it departs. The temperature drop between the incoming and outgoing water depends on the flow rate. Running full open the water temp only drops a few degrees. But running only one sprinkler and so reducing the flow, the temp difference can be as much as 15 degrees. The inlet hose will be wet with condensation but the out hose will be completely dry. Interestingly the fan speed doesn't appear to have to much affect on the difference in the water temp, but it does have an effect on the air temp. The lower the fan speed the colder the air coming out of the cooler. That's because the air spends a little longer moving over the fins and gets a chance to shed more heat. However the cold air doesn't circulate around as much because there isn't much air movement, so I usually run the fan on high or medium.
Also in case anyone is wondering or interested, my well is around 70 feet deep with a submersible pump at the bottom and a pitiless connector about 10 feet down. Because our ground freezes down to 8 feet on occasions all underground pipes need to be at least that deep if not deeper to prevent them from freezing. So this is a sealed well, the top of the casing is closed to prevent any contamination from getting into it. Our water table is around 20 feet down so the pump is well below the table. This allows for it to draw down a lot before it starts sucking air.