SynthTest allows you to pick a MIDI source, a MusicDevice AudioUnit and an Effect AudioUnit to play with.
There is no purpose to SynthTest anymore. Use Apple's AU Lab.app that comes with the developer tools instead. It is much more featureful, with support for multiple tracks and audio I/O.
The source code to SynthTest is ancient, and will not compile as is. You're on your own with this source; I can't support it anymore.
This version fixed crash related to and added workaround for audio units that do not have a space after the colon in their combined name.