TouchOSC - Modular OSC and MIDI control surface for iPhone / iPod Touch / iPad / Android
Soundplant - Play sound files with the computer keyboard
Open Stage Control - Libre and modular OSC / MIDI controller
CasterSoundboard - A soundboard for hot-keying and playing back sounds. (For podcasting)
LIVKONTROL - LK is a touchscreen Ableton Live and MIDI controller app for iOS and Android tablet and mobile devices.
JN Soundboard - An open-source hotkey soundboard program