If you aren't necessarily focused on tying hardware controllers to a software app (which limits you to MIDI stuff), you might consider one of the basic programm

If you aren't necessarily focused on tying hardware controllers to a software app (which limits you to MIDI stuff), you might consider one of the basic programmable microcontrollers, like the Parallax BASIC Stamp: http://www.parallax.com/ http://www.parallax.com/ It is designed to be easily plugged into devices, gives you a simple programming language, and a way to interface with a computer. I bet you can snag one on Ebay for cheap.