Mac OS X boot-time keyboard shortcuts

Mac OS X boot-time keyboard shortcuts (in no particular order):

1. Hold down D during startup
   • Takes you to Apple's Hardware Test utility
2. Hold down Alt+Cmd+P+R during startup
   • Clears the PRAM (parameter RAM) and NVRAM (non-volatile RAM): if either the PRAM or the NVRAM become corrupt, problems will ensue. Hold the keys down until your computer restarts: release the keys after you hear the second startup sound.
3. Hold Alt down during startup
   • This will load the boot menu allowing you to boot to a different operating system partition you may have installed on your hard drive. May also allow you to boot to a USB, but that's just a conjecture.
4. Hold down N during startup
   • if your setup includes a compatible network server (NetBoot), you can hold down the said key during boot to attempt a network boot. You can alternatively use the Alt+N keys during startup to start from a NetBoot server using the default boot image provided.
5. Hold down T during startup
   • puts your Mac into Target Disk Mode, effectively turning your computer into a large removable flash drive. (Allows you to transfer files to and from your machine with...ease?)
6. Hold down Shift during startup
   • Boot into safe mode. Very similar to Windows Safe Mode; a great way to troubleshoot OSX applications and extensions that aren't working properly.
7. Hold down Cmd+V during startup
   • Boot into verbose mode. A terminal-like interface will appear while booting. It will contain information important to startup, allowing you to diagnose startup problems by seeing any errors that may be occurring during startup. Verbose mode exits automatically when your mac finishes booting.
8. Hold down Cmd+S during startup
   • Boot into single user mode.
     NOTE A knowledge of bash and the command-line (in general) is recommended. Handy for advanced startup sequence troubleshooting.