AU Lab is a digital mixer application which hosts Audio Unit plugins. It's part of Xcode Tools and is included on the Tiger install DVD, although not installed by default. (Xcode Tools can also be downloaded from the Apple Developer Connection website, which requires (free) membership.)

Although intended for programmers, there's nothing about it which should frighten off the average mac music maker! Pretty much like Rax, really, but configurable; allows you to build your own mixer with inputs, outputs, instruments, effects and busses; it records, recieves midi, generates midi clock, and offers a performance meter.