OS X Startup keys for Intel Macs

• 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 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 N during startup - Attempt to start up from a compatible network server (NetBoot).
• Press T during startup - Start up in FireWire Target Disk mode. (explanation: your mac behaves as a very expensive firewire external hard drive in this 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-Command-P-R until you hear two beeps - Reset NVRAM
• Press Eject, F12, or hold the mouse (/trackpad) button - Ejects any removable media, such as an optical disc.
• Press Option-N during startup -
  Start from a NetBoot server using the default boot image.

Courtesy of the Apple website.