Reset the PRAM
Hold down the Command+Option+P+R keys during the restart sequence. The computer will beep for a normal startup and will beep again. When you hear the second beep, release the keys.

Turn Off Extensions
Hold down the Shift key at startup until you see the Welcome to Macintosh screen.

Rebuild the Desktop
Start or restart the computer while holding down the Command-Option keys.

Disable Virtual Memory
Hold down the Command key at startup. Do not release the key until startup is complete.

Extension Manager
Bring up the Extension Manager at startup by holding down the Space Bar.

Restart/Shutdown Color
Restart and shut down the computer by holding the Command + Option + Shift + Power key

Force Reboot
Force reboot by holding the Command + Control + Power key

Startup From Bootable CD-ROM
Hold down the letter "c" key upon boot (lower-case only).

Eject floppy (used for external USB drives)
Hold down the mouse button on boot.