CasterSoundboard - A soundboard for hot-keying and playing back sounds. (For podcasting)
Keezy Drummer - Make a beat in seconds
JN Soundboard - An open-source hotkey soundboard program
Sampulator - Make (and record) beats on your keyboard
Soundplant - Play sound files with the computer keyboard
Auxy Music Studio - Auxy brings real music creation to iPhone in a simple and inspiring format.