Boot keyboard combinations for Macs

List of keyboard combinations to use while you "Intel-based" Mac is booting up.

For best results use these key combinations right after you hear startup sound.

Key Combination	Description
C	Boot from a bootable CD or DVD, such as the Mac OS X Install disc or even a Linux LiveCD
D	Apple Hardware Test (AHT), the Install DVD 1 must be in optical drive.
Option-Command-P-R	Hold until you hear two beeps to Reset NVRAM
Option	Startup/Boot Manager, allows you to select a Mac OS X volume, BootCamp, External or Optical Drive.
Eject or F12	Ejects any removable/optical media from drive. Also works by holding down the mouse/trackpad button
N	NetBoot (Network Boot)
T	FireWire Target Disk mode. Allows you to mount machine on other Mac as an external drive
Shift	Safe Boot mode. Temporarily disables login items.
Command-V	Verbose mode.
Command-S	Single-User mode.
Option-N	NetBoot using default boot image.

More Information: Apple KB Article HT1533