This is interesting way of using a PS2 controller. Curious Inverter has constructed a PIC based PS2 game controller in to MIDI controller. PIC18f4550 microcontroller firmware does button remaping to different notes and functions.
This controller can be built by anyone with intermediate electronics skills. Circuit includes only interface connectors while other functions are performed in firmware. In case you want complete device you can order kit or fully assembled (and tested) construction and enjoy being a MIDI or guitar hero.