The following is a collection of what I have found on the Net.

• C : Forces most Macs to boot from the CD-Rom drive instead of the internal hard drive. Only works with Apple ROM drives and with bootable CD discs.
• D : Forces the first internal hard drive to be the startup disk.
• N : Netboot (New World ROM machines only) - Looks for BOOTP or TFTP Server on the network to boot from.
• R : Forces PowerBooks to reset their screen to default size (helpful if ve been hooked up to an external montior or projector!)
• T : Target Disk Mode (FireWire) - Puts machines with built-in FireWire into target Disk mode so a system attached with a FireWire cable will have that device show up as a hard drive on their system. Very useful for PowerBooks!
• Mouse Button Held Down : Ejects any mounted removable media.
• Shift : Disables all extensions (Mac OS 7-9), or disables Login items when using Mac OS X 10.1.3 or later. Also works when booting Classic mode up just like you were using the OS natively.
• Option : When using an Open Firmware "New World ROM" capable system, the System Picker will appear and query all mounted devices for bootable systems, returning a list of drives & what OS they have on them. On "Old World" systems the machine will simply boot into s default OS without any Finder windows open.
• Space bar : Brings up s Extension Manager (or Casady & s Conflict Catcher, if installed) up at startup to allow you to modify your extens ion set.
• Command-V : Boots Mac OS X into "Verbose Mode", reporting every console message generated during startup. Really shows s going on behind the scenes with your machine on startup!
• Command-S : Boots Mac OS X into "Single User Mode" - helpful to fix problems with Mac OS X, if necessary.
• Command-Option : Rebuilds the Desktop (Mac OS 7-9).
• Command-Option-P-R : Erases PRAM if held down immediately after startup tone. Your machine will chime when s erased the PRAM, most people will hold this combination for a total of 3 chimes to really flush the PRAM out.
• Command-Option-N-V : Erases NVRAM (Non-Volatile RAM). Used with later Power Macintosh systems mostly.
• Command-Option-O-F : Boots the machine into Open Firmware (New World ROM systems only).
• Command-Option-Shift-Delete : Forces your Mac to startup f rom its internal CD-ROM drive or an external hard drive. Very helpful if you have a 3rd party CD-ROM drive that is not an Apple ROM device.
• Command-Option-Shift-Delete-#(where #= a SCSI DEVICE ID) : Boot from a specific SCSI device, if you have your 3rd party CD-ROM drive set to SCSI ID 3, you would press "3" as the # in the combination.