Download of SynthTest 1.2.1 for Mac was on the developer's website when we last checked. We cannot confirm if there is a free download of this app available. The software is included in Audio & Video Tools.
Please check the Mac app with an antivirus before launch as it is downloaded from the developer's website, and we cannot ensure that it is safe. The most popular version among the program users is 1.2. This free software for Mac OS X was originally produced by Chris Reed.
From the developer:
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.
You may want to check out more Mac applications, such as V-Synth Librarian, M-Audio Key Rig or VirtualDeckBrowser, which might be similar to SynthTest.