engadget found a great tutorial on making your own foot pedal from an old USB keyboard for relatively cheap (assuming you can solder). Mostly it’s great because I’ve always wanted some foot pedals, but they’re pretty expensive. It’s unclear what to do with the wires in the foot pedal (i.e., how to make the pressing connect the circuit) but the comments elaborate.
The downside of this tutorial is that it’s for specific keys only and takes a little know-how. If you’re running Windows, you can couple this with AutoHotKey to map whichever key you choose to various actions (or other keys). If you map it to something you rarely use (say caps lock) and you’re running Windows, you can use SharpKeys as a frontend to editing the keymap. I have my normal caps lock key remapped to an F-key, which I can now use in any program that gives you control over the keyboard commands (games especially).
I think I’ll try making a few of these next chance I get, setting them to F-keys, and binding them to all sorts of interesting OS X things (for instance, I’ve been using a Logitech mouse with a button mapped to Expose for a while).