If you’re an Apple Garageband user, you may want to download midiO, an Audio Unit plugin for DAWs (like GarageBand) that don’t support MIDI output.

The plugin lets you drive a MIDI-based synthesizer from any DAW that supports software instrument Audio Units.

Version 1.3, posted on 9/20/06: Passes AU validation under 10.6. Can generate program and bank change messages.