There are three problems I have with these toys, though: First, they're pretty expensive. Browsing around, it looks like you're paying about 80 cents to a dollar per strut (like this). Second, they're limited to regular or semi-regular geometry because all the struts are the same size. Third, the structures you build wind up pretty small because the struts are short. This means that the models you build don't translate very well to something economical to build in real life.
This Instructable will outline my solution with a low-cost, custom-length, scalable magnetic construction kit you can build without special tools and made from parts that are easy to get (including regular ol' drinking straws!).
Why not build your own building kit?
Step 1: Stuff You Can Model
Since the store-bought kits are usually all the same length, they're great for making regular geometric forms. You can wireframes of the Platonic Solids in this way.
Everyone loves the iconic geodesic dome. These wonderful architectural structures look like they're made up of equilateral triangles, but the lengths of the struts vary slightly to produce the spherical curvature. Desert Domes has a great dome calculator tool that will help you determine the lengths you need. You can't make a true geodesic dome with the store-bought kits.
But why limit yourself to regular geometry? Vary the lengths of struts as you wish and create free-form structures on all sorts of scales, from furniture to sculpture to large architectural features. I made a spaceframe bench, and a roof truss for a temporary classroom (see pictures) with hubs and struts. You could even make a model that could become your house. (This paper is somewhat related, super cool and futuristic).
It's time to use your imagination!
Step 2: Gather Materials
Drinking straws come in different diameters. There are really tiny ones for stirring your coffee, super large ones for drinking bubble tea, big ones you get at restaurants like McDonalds where they want you to guzzle down soda like there's no tomorrow, and "regular" straws of medium diameter. The standard diameter of the "regular" straws is a seemingly-arbitrary 0.22".
All the magnets I can easily get come in fractional inch sizes. Unfortunately, there is no fractional inch size that's equivalent to 0.22". 1/4" is 0.25", but a round magnet of that size doesn't fit inside the straws. Fortunately a square 3/16" magnet works nearly perfectly. Why? Because the perimeter of the square magnet is slightly larger than the circumference of the round straw, but not too much larger. The square magnet is stretching the plastic of the straw 60% of what a 1/4" round magnet would require. This is the best fit I've found - snug, but not impossible to insert.
circumference(round straw) = 0.22 * pi = 0.69"
circumference(round 1/4" magnet) = 0.25 * pi = 0.79" (Way too big!)
perimeter(square 3/16" magnet) = 0.1875 * 4 = 0.75" (Just too-big enough!)
The steel spheres serve as the hubs of your structures - the place where struts come together. They should be big enough to accommodate many struts, but not so big that the structure is unnecessarily heavy and expensive. I find that 3/8" is pretty good for this size straw, though 1/2" might be good for really complicated stuff with lots of tight corners and many struts at a hub.
Please comment if you experiment and find other combinations that work for you! I haven't had a chance to experiment much with different size straws.
Different kinds of drinking straws are made from different plastics. Try a few to see what you like; I've found stronger ones that are more brittle (they'll crack if you bend them too much) and weaker ones that are much more resilient. You also have choices in terms of colors.
Where to Buy
Magnets - K&J Magnetics is a good place to shop. They ship quickly and reliably, with decent prices, quality magnets and good info. The 3/16" square magnets I use are product number B333. You can find cheaper stuff on eBay, but you probably get what you pay for in quality and magnet strength. Then again, stronger magnets aren't always a good thing here - they can increase the risk of the magnet pulling out from the straw. Cost from K&J is about 16-20 cents depending on quantity.
Straws - Get these locally; you'll have a chance to feel the strength, see colors, and compare a few options quickly. Grocery stores, party supply shops, and big-box retailers like Wal-Mart and Target are good sources.
Spheres - K&J sells these with a nickel plating on them. That means they won't rust. However, they're significantly more expensive than plain steel ones you can get from McMaster Carr. A pack of 250 will cost you a mere $5.70. That's less than 3 cents each (They're about 15 cents on K&J)! I've been playing with mine for a while and, despite being uncoated, they show no signs of rust. If they eventualy do, they're cheap enough to replace.
The total cost of each strut will be about 40 cents if you buy a reasonable quantity of magnets (2 magnets per strut). That's about half of what the commercial kits cost!
The only tools you really need to make your construction kit are scissors and optionally a ruler.
Step 3: Measure and Cut Straws
If you just want to explore, cut a bunch of random straw lengths. Maybe include a bunch of uniform length straws of the same color as well.
Regular scissors seem to work best for this. Maybe a paper cutter would let you chop a whole bunch at once; I haven't tried this yet.
Step 4: Insert Magnets Into Straw Ends
Turn the magnets around when inserting into the other side of the straw. This ensures that each straw has both a north and south pole exposed, making it more likely to find a happy orientation within your structure. You might find that you need a strut with two north or two south poles instead; it's easy enough to pull the magnet back out and flip it around if needed.
Simple! No injection molding required!
Step 5: Start Building!
I'd love to hear about your creations in the comments!
(If you're interested in doing more experiments with your model, check out this Instructable I wrote on how to incorporate force sensors into your structure).