Shortcuts for starting, restarting, logging out, and shutting down Mac OS X

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.)

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88 is Command
Reference : Mac OS X in a Nutshell