SynthTest is a simple application for OS X that allows you to pick a MIDI source, a MusicDevice AudioUnit and an Effect or MusicEffect AudioUnit to play with. You can open an edit window for both the synth and effect. If they do not have a custom edit GUI, the generic editor view will be used.
It is primarily intended to be used by Audio Unit developers for testing purposes, although it can be fun to play with all by itself. In the end, I would like SynthTest to become an open reference implementation of an AudioUnit host.