For those who may not know about it, Apple includes a powerful AU plugin host called AU Lab when you install the XCode Development package. It is worth having if you need a good mixer and host for software instruments and effects. It allows you to build your own mixer with inputs, outputs, instruments, effects and busses. It also records, receives midi, generates midi clock, and offers a performance meter.
You can find the XCode developer tools on your Mac OS X Install DVD. Once installed you will find AU Lab on your system drive at /Developer/Applications/Audio/AU Lab.
