This article is from the Mac Programming FAQ, by Jon Watte firstname.lastname@example.org with numerous contributions by others.
MacsBug is freely available for ftp from <ftp://ftp.apple.com>; log
in as user anonymous and give your FULL e-mail address as password.
MacsBug is your basic monitor-type debugger that takes a few hundred Ks
of memory, and lets you break, step, disassemble, look at the stack etc
of most anything running on your Mac. Since it's free (it's also on the
developer CDs) and provides most of the functionality you need, this is
a popular choice. As of 6.5d10, Macsbug supports PowerPC debugging.
Jasik Designs has a debugger called The Debugger which can do both low-
and high-level debugging, with or without source and for all types of
code, application, code resources, everything. This is the debugger of
choice for many large developers because of its high power and many
features not found anywhere else. However; newcomers beware! This is
the Lamborghini of debuggers; if you know how to drive it, it is the
fastest way from A to B; if you don't, you'll just end up in the ditch.
The Debugger is PowerMac native and supports PowerPC disassembly. It
includes an excellent code coverage tool and MacNosy, a general
disassembler. Support is direct from the author and generally great.