When booting, hold these to change how the machine starts up:

Cmd-V ---- Verbose mode -- displays what's actually occurring while OS X boots up

Cmd-S ---- Boots into single-user mode (which provides a Unix shell as the root user).

Shift ---- When held at boot, will disable any kernel extensions. When held when logging-in, will disable any user-specific start-up items.C ---- Boot from a CD

D ---- Boot using the first partition

N ---- Boot from a network server

R ---- Reset the screen on a PowerBook

T ---- Start in Target mode (to use a machine as if it was a FireWire hard drive)

X ---- Boot in OS X (if OS 9 was the previous OS)

Option ---- Show a list of possible startup disk options

Cmd-Opt-Shift-Delete ---- Boot from an external drive or CD/DVD-ROM

Cmd-Opt-P-R ---- Zap the PRAM (Will restart and chime again. Do at least 4 times!)

Cmd-Opt-O-F ---- Boots into Open Firmware