Mac.

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

Mac Intel

Press C during startup Start up from a bootable CD or DVD, such as the Mac OS X Install disc that came with the computer.

Press D during startup Start up in Apple Hardware Test (AHT), if the Install DVD 1 is in the computer.

Press Option-Command-P-R until you hear two beeps. Reset NVRAM

Press Option during startup Starts into Startup Manager, where you can select a Mac OS X volume to start from. Note: Press N to make the the first bootable Network volume appear as well.

Press Eject, F12, or hold the mouse button Ejects any removable media, such as an optical disc.

Press N during startup Attempt to start up from a compatible network server (NetBoot).

Press T during startup Start up in FireWire Target Disk mode.

Press Shift during startup Start up in Safe Boot mode and temporarily disable login items.

Press Command-V during startup Start up in Verbose mode.

Press Command-S during startup Start up in Single-User mode.

Press Option-N during startup Start from a NetBoot server using the default boot image.