Besides the options outlined in the previous post, Macs also support the following boot time options
Press Option-Command-P-R until you hear two beeps. Reset NVRAM.
Press Eject, F12, or hold the mouse button Ejects any removable media, such as an optical disc.
Press N during startup Attempt to start up from a compatible network server.
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.