MidiKeys

MidiKeys is a small application for Mac OS X that presents a miniature MIDI keyboard onscreen. You can use the computer keyboard to play MIDI notes, or click on the keys with the mouse. There is also an option to select a MIDI source and see incoming notes played on the keyboard.

MidiKeys has no way to produce sound on its own. In order to hear a sound when you press keys, you need to connect the MidiKeys output to a synthesizer of some sort. This can be either a softsynth or a hardware synth connected through a MIDI I/O box. Apple's AU Lab works nicely to play Audio Units.

Note that MidiKeys requires 10.5 to run.