The Macintosh equivalent of control-alt-delete is command-control-power. (Isn't that great? "Command! Control! Power! -- Bring me more; I yet lust for keys!")
Before resorting to that, however, try command-option-escape to bring up the force quit dialog, which sometimes gets rid of the hung application, sometimes gets rid of the hung application but releases bogons into your system, and sometimes doesn't work at all. If you're deserving, the system will be stable enough to draw the dialog naming the offending application, which might, e.g., clue you in to a software conflict.
If you're a hard-core Mac geek with a steady hand and a flinty gaze, try hitting command-power at your next Silent Screen. The very core of the OS will belch up a crude command-line debugger, drawn in an itty-bitty window, at which you can type "g finder" and hit return for unpredictable but occasionally beneficial results. (If you're cool, you've installed MacsBug, which replaces that crude debugger with a merely primitive one.)
If you don't want to remember this stuff, (a) install MacOS X, (b) install Yellow Dog or another PPC Linux, or (c) choose a date not too long after that of your Mac's manufacture and install no software released after it. Otherwise, accept that MacOS Classic, splendid creature though it is, ain't nohow a stable operating system.
The comic strip Sally Forth once referred to the chord
command-option-backspace. It was very possibly the most sad, stupid, pathetic thing ever.