This article is from the Macintosh system software FAQ, by Elliotte Harold firstname.lastname@example.org with numerous contributions by others.
On Macs that don't have a physical programmer's switch you
can restart the computer with Command-Control-Power and drop
into the debugger with Command-Power. Also note that in System 7
Command-Option-Escape will force most applications (including
the Finder) to quit so you no longer need to activate the debugger
just to kill a frozen application.