TouchOSC - Modular OSC and MIDI control surface for iPhone / iPod Touch / iPad / Android
UVI Falcon - Sampler/synth hybrid instrument
Open Stage Control - Libre and modular OSC / MIDI controller
Pianoteq - Pianoteq is a virtual piano that runs on your computer.
LIVKONTROL - LK is a touchscreen Ableton Live and MIDI controller app for iOS and Android tablet and mobile devices.
LinuxSampler - LinuxSampler - free, open source pure software audio sampler with professional grade features.