XO Wave is a basic multi-track audio tool with multi-channel recording and mixing, video support, plug-in support (in the Pro version) and built in DSP, double-precision math, and non-destructive editing. It looks like it could be a strong choice for basic multichannel tasks. And it has some less-common features, like automatic softening to remove clicks/pops at edit points, and versioning so you can go back to earlier versions of files. A very capable version is available free, and a “Pro” version is just US$95 (though that admittedly puts it in slightly more competitive waters).
Interestingly, this is also one of the rare cases of a Java-based audio app. (The app is Java-based, at least; the developer notes that audio processing is not done in Java.) The 1.0 final release is compatible with Mac OS X Leopard, with two caveats: one, 10.5′s new security privileges cause it to gripe the first time you run it about security (as it would with any app), and two, dock/switcher icons appear twice. (Java support on 10.5 has a couple of hiccups; at least they’re non-critical annoyances; the icon issue is apparently a Leopard problem, not Java per se.)
1.0 has also arrived on Linux; in that version the software is free (though closed-source, despite the name, with full JACK support). (Hey, how about a JACK-aware Mac version, too?)