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Command-S Boot into Single User Mode
Command-V Boot using "Verbose" mode (shows all kernel and startup console messages)
X Reset startup disk selection and boot into Mac OS X Server
Shift Boot into "Safe Boot" mode, which runs Disk First Aid. A reboot will be required afterward.
Option Boot into Open Firmware to select a boot device
Command-Option-Shift-Delete Bypass internal harddrive on boot
T Boot into Firewire target disk mode
C Boot from the internal optical drive
N Start from the Network (NetBoot)
Command-Option-P-R Reset Parameter RAM (PRAM) and non-volatile RAM (NVRAM)
(mouse button) Eject (internal) removable media

ALSO: if you use open firmware password... you'll need this:
Startup Manager -accessed by pressing the Option key during startup
Enter commands after starting up in Open Firmware -press Command-Option-O-F key combination during startup.

http://docs.info.apple.com/article.html?artnum=106482

How to troubleshoot a computer with Open Firmware Password enabled
If you cannot access the Open Firmware Password application and need to troubleshoot your computer by:

Resetting the PRAM
Starting up in Single-user mode
Starting up in Verbose mode
Starting from CD-ROM

Then follow these steps:

Start up into Open Firmware by pressing and holding the Command-Option-O-F key combination during startup.
At the Open Firmware prompt, type: reset-nvram
Press Return.
When prompted for your password, enter it and press the Return key. It responds OK.
At the Open Firmware prompt, type: reset-all
Press Return.

The computer restarts and you are now be able to reset the PRAM and startup in Single-user mode, Verbose mode, or from CD-ROM.

Startup
Keystroke Description
Press X during startup Force Mac OS X startup
Press Option-Command-Shift-Delete
during startup Bypass primary startup volume and seek a different startup volume (such as a CD or external disk)
Press C during startup Start up from a CD that has a system folder
Press N during startup Attempt to start up from a compatible network server (NetBoot)
Press R during startup Force PowerBook screen reset
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 and non-essential kernel extension files (Mac OS X 10.2 and later)
Press Command-V during startup Start up in Verbose mode.
Press Command-S during startup Start up in Single-User mode (command line)