C : Holding down the C key at startup will boot from a CD (useful when installing or upgrading the system software).
T : Holding down the T key at startup will boot from a FireWire drive, if it has a bootable System folder.
X : Holding down the X key at startup will force the machine to boot into Mac OS X, even if Mac OS 9 is specified as the default startup disk.
88-S : Boots into single-user mode.
88-V : Boots into verbose mode, displaying all the startup messages onscreen. (Linux users will be familiar with this.)
Shift : Holding down the Shift key at startup invokes Safe Boot mode, turning off any unnecessary kernel extensions (kexts), and ignoring anything you’ve set in the Login Items preferences panel.
Option : Holding down the Option key at startup will take you to the Startup Manager, which allows you to select which OS to boot into.
Mouse button : Holding down the mouse button at startup will eject any disk (CD, DVD, or other removable media) that may still be in the drive.
Shift-Option-88-Q or Option + Apple –> Log Out : Logs you off without prompting you first.
Option-Power-on or Option + Apple –> Shut Down : Shuts down your system without prompting you first.
Option-Apple–>Restart : Restarts your machine without prompting you first.
Control-88-Power-on button : Forces an automatic reboot of your system; this should be used only as a last resort as it could mess up your system. (Mostly, you’ll just wait forever at the gray Apple startup screen while an fsck happens in the background.)
88 is Command
Reference : Mac OS X in a Nutshell