Magic Music Visuals - Fully-customizable and highly responsive visuals for live and recorded audio & MIDI.
TouchOSC - Modular OSC and MIDI control surface for iPhone / iPod Touch / iPad / Android
MilkDrop - 3D audio visualization plugin, compatible with WinAmp plugin API
Open Stage Control - Libre and modular OSC / MIDI controller
projectM - projectM is an LGPL'ed reimplementation of Milkdrop under OpenGL.
LIVKONTROL - LK is a touchscreen Ableton Live and MIDI controller app for iOS and Android tablet and mobile devices.