Lists of Free Audio Units plugins for Mac OSX

For DAW like GarageBand, Logic, Ableton Live, SoundTrack Pro, Final Cut, AULab

Audio Units (AU) are a system-level plug-in architecture provided by Core Audio in Mac OS X developed by Apple Computer. Audio Units are a set of application programming interface services provided by the operating system to generate, process, receive, or otherwise manipulate streams of audio in near-real-time with minimal latency. It may be thought of as Apple's architectural equivalent to the other popular plug-in format, Steinberg's VST. Because of the many similarities between Audio Units and VST several commercial and free wrapping technologies are available (e.g. Symbiosis and FXpansion VST-AU Adapter). (Wikipedia)

Air Windows
Camel Crusher
Sample Tank Free
OHM Force
TAL plugins
Addictive Drums

(Where you can find more AudioUnits)
List and Description of Free AudioUnits
Ten free plugins (AU and VST) review
KVR Search for AUs

Wikipedia article about Audio Units
Audio Unit Programming Guide