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 N during startup - Attempt to start up from a compatible network server (NetBoot).
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.