MidiKey is a cute program that can make your mac keyboard become an instrument keyboard. you can play 128 instruments on your Mac.it is able to receive MIDI input from MIDI keyboard. also can send MIDI OUT to native applications such as GarageBand, Logic Pro, Pd...
Some Tips:
Press Space Bar to set a random instrument (In 128 General MIDI sounds),
Press Up Arrow Key or Down Arrow Key to change octaves,
Press Left Arrow Key or Right Arrow Key to change pitch,
Hold on Shift key to play accent notes