This is a hardware hint, but it saves me work each time I use it, so I figured it was worth sharing.
If a Mac dies due to a bad system or such, and (as is quite frequent on the tray loading iMacs) the CD-ROM drive has been vandalised, you're locked out. The only option is to remove the HD, mount it in a working Mac, and repair. Just digging a hard drive out, though, can be a time-consuming pain. But if you can get to the interface cable and plug in an IDE to USB cable instead, a repair can be effected in situ.
[robg adds: I added the froogle search link, just to show the item under discussion.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050122035745985