You can use the following startup key combinations with Intel-based Macs.

Press C during startupStart up from a bootable CD, DVD, or USB thumb drive (such as OS X install media).

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Press D during startupStart up in Apple Hardware Test (AHT).

Press Option-Command-P-R until you hear startup sound a second time.Reset NVRAM

Press Option during startupStart up in Startup Manager, where you can select an OS X volume or network volume to start from.  

Press Eject, F12, or hold the mouse or trackpad buttonEjects any removable media, such as an optical disc.

Press N during startupAttempt to start up from a compatible network server (NetBoot).

Press T during startupStart up in Target Disk Mode.

Press Shift during startupStart up in Safe Boot mode and temporarily disable login items.

Press Command-V during startupStart up in Verbose mode.

Press Command-S during startupStart up in Single-User mode.

Press Option-N during startupStart from a NetBoot server using the default boot image.

Press Command-R during startup

Start from the OS X Recovery System1

Available on Macs that ship with OS X Lion. Some CPUs require an EFI BootROM update to support this feature.