CasterSoundboard - A soundboard for hot-keying and playing back sounds. (For podcasting)
TouchOSC - Modular OSC and MIDI control surface for iPhone / iPod Touch / iPad / Android
JN Soundboard - An open-source hotkey soundboard program
Open Stage Control - Libre and modular OSC / MIDI controller
Soundplant - Play sound files with the computer keyboard
Lemur - Lemur, the professional MIDI and OSC controller app that doesn't cut corners, has evolved.