Virtual MIDI Piano Keyboard - Virtual MIDI Piano Keyboard is a MIDI events generator and receiver.
Midi Studio - Full-featured, modular MIDI control surface for iOS
Free Piano - FreePiano is a virtual MIDI keyboard and a VST host for windows.
Navichord - Intuitive chord sequencer and MIDI controller for iOS
MidiKeys - MidiKeys is a small application for Mac OS X that presents a miniature MIDI keyboard onscreen.
IsoPad - IsoPad lets you use your Windows tablet to play electronic MIDI instruments.