X during startup - Force Mac OS X startup
option - (eventually) brings up a screen with startup volume choices
Option-Command-Shift-Delete - Bypass primary startup volume and seek a different startup volume (such as a CD or external disk)
C - Start up from a CD that has a system folder
N - Attempt to start up from a compatible network server (NetBoot)
T - Start up in FireWire Target Disk mode (very handy for plugging your Mac into another as an external hard drive)
Shift - start up in Safe Boot mode and temporarily disable login items and non-essential kernel extension files (Mac OS X 10.2 and later)
Command-V - Start up in Verbose mode.
Command-S - Start up in Single-User mode
cmd-opt-p-r - Zap PRAM. Hold down until second chime.
cmd-opt-n-v - Clear NV RAM. Similar to reset-all in Open Firmware.
cmd-opt-o-f - Boot into open firmware
hold mouse click - force eject a CD/DVD
(Via The Unofficial Apple Weblog (TUAW).)
I might need these for future reference. For instance, if you ever want to boot from a linux live cd/dvd or something.
Tags: boot, mac