Startup Keys

This was found in the Mac help file:

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 (/trackpad) 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.

 

There are many sources for this including, as osiris points out, the built-in Help. Apple also has a page. Here is a rather complete list of keyboard shortcuts.