Update March 8: I added pictures for the fuse and switched to using a SSR.
This will show you how to build your own accurate temperature controller for cooking with sous vide. How much you spend depends on what you use for a heating element. This controller is accurate to 0.1° C and is able to keep a steady temperature within 0.2°C of the desired temperature.
First I need to give credit to Scott at Seattle Food Geek. This instructable is inspired by his genius DIY sous vide. Check it out for an alternative to mine at http://seattlefoodgeek.com/2010/02/diy-sous-vide-heating-immersion-circulator-for-about-75/
My controller is a little more generic than Scott's in that it can use any electric heat source to keep the water bath at a constant temperature. I use a cheap crock pot but plan on using a bucket heater in a cooler when cooking for large parties.
Let me know if there are any mistakes of missing steps and I'll try to fix them asap.
Step 1: Parts list
1) PID controller. I got mine on Ebay for ~$35 (including shipping). Most controllers will do but avoid ones that have a built-in relay--the relays usually aren't rated for the current your heater(s) will use and will blow out your PID controller. The wiring for this instructable assumes your controller puts out 12v to control an external relay.
2) Aquarium pump for water circulation. You don't have to get too fancy--it seems that most pumps can handle the heat for common sous vide cooking (< 70° C).
3) 10+ amp electric relay. I originally used a mechanical relay (see pictures) but it eventually wore out. I've updated the build with a 10 amp SSR that I got used on EBay for $4 (many people on Scott's page are using 25 amp SSRs). If you don't know anything about SSRs make sure you get one with DC control voltage and AC output voltage. This one will do: http://www.amazon.com/dp/B004HZN628/ Depending on what you're using for a heat source you may want to get a heat sink. See my discussion in the step that installs the SSR.
4) 10+ amp switch. I bought a light switch from Home Depot like the following (mine was cheaper): http://www.homedepot.com/h_d1/N-5yc1vZ1xh3/R-100356855/h_d2/ProductDisplay?langId=-1&storeId=10051&catalogId=10053 . Get a rectangular one because it'll be easier to cut the holes in your enclosure.
5) 10+ amp dual socket like this one: http://www.homedepot.com/h_d1/N-5yc1vZ1xh3/R-100117108/h_d2/ProductDisplay?langId=-1&storeId=10051&catalogId=10053 . Again, get a square one because it'll be easier to install.
6) A 3-prong power cable with bare leads.
7) A PT100 temperature probe (thermocouple). Get a three-wire probe because it'll be a little more accurate (and because I don't know how to hook up a two-wire probe to the PID controller).
8) Some sort of plastic enclosure. I went with a junction box because they're easy to cut with a Dremel and very sturdy but at $11 it's overkill. You can get a plastic acrylic box for ~$4. I'd advise against a metal box to avoid electric shorts.
9) 10 amp fuse and fuse holder.
10) Various wires and screws. For doing the AC power wiring you want wire rated for 10 amps.
11) 8-screw terminal block.
12) Electric heat source. A crock-pot works well. Make sure you get one that turns on with a mechanical switch--the controller is going to continuously toggle the power to the heat source and you don't want a digital crock pot that has to be reset after every power cycle. I got mine at a thrift store for $15.
Not required but handy:
1) An outlet wallplate to use as a template when cutting your enclosure. (Like this: http://www.homedepot.com/h_d1/N-5yc1vZ1xh3/R-202059861/h_d2/ProductDisplay?langId=-1&storeId=10051&catalogId=10053 .)
2) LED for indicating when the heater is on. Most PID controllers have a little light but I wanted mine to be obvious. I got an LED assembly with a built-in resistor.
3) Male and female three-wire audio jack for hooking up the PT100 thermocouple.