I’ve been doing some troubleshooting lately and find this list of startup key commands very helpful:

Mac

• X during startup – Force Mac OS X startup
• option – (eventually) brings up a screen with startup volume choices
• Option-Command-Shift-Delete – Bypass primary startup volume and seek a different startup volume (such as a CD or external disk)
• C – Start up from a CD that has a system folder
• N – Attempt to start up from a compatible network server (NetBoot)
• T – Start up in FireWire Target Disk mode (very handy for plugging your Mac into another as an external hard drive)
• Shift – start up in Safe Boot mode and temporarily disable login items and non-essential kernel extension files (Mac OS X 10.2 and later)
• Command-V – Start up in Verbose mode.
• Command-S – Start up in Single-User mode
• cmd-opt-p-r – Zap PRAM. Hold down until second chime.
• cmd-opt-n-v – Clear NV RAM. Similar to reset-all in Open Firmware.
• cmd-opt-o-f – Boot into open firmware
• hold mouse click – force eject a CD/DVD

Mac Intel

• 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 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.