midiStroke allows you to trigger keystrokes in the currently focused application using MIDI (Musical Instrument Digital Interface) note, program and CC messages. Each MIDI message can trigger an unlimited number of keystrokes in sequence. This is of enormous value for performing electronic musicians, but can also be used to automate everyday tasks through the use of MIDI footpedals, keyboards and control surfaces. With the 1.1 update, you could even use a keyboard MIDI controller as a complete computer keyboard replacement.
midiStroke runs on Mac OSX and was programmed with a combination of C and Objective-C. You can read more about it in a couple of recent magazine articles: Electronic Musician and Remix.