TouchOSC - Modular OSC and MIDI control surface for iPhone / iPod Touch / iPad / Android
Kontakt - With KONTAKT 6, you can sound like a snare drum, a symphony orchestra, or anything in between, from a universe of sampled instruments.
Open Stage Control - Libre and modular OSC / MIDI controller
UVI Falcon - Sampler/synth hybrid instrument
LIVKONTROL - LK is a touchscreen Ableton Live and MIDI controller app for iOS and Android tablet and mobile devices.
Pianoteq - Pianoteq is a virtual piano that runs on your computer.