Since they're in firmware, the following don't care whether the machine is booting Mac OS X or OS 9.
Option: show startup manager (to choose boot volume on the fly)
Command-Option-P-R: reset PRAM/NVRAM
Command-Option-O-F: enter Open Firmware
Command-Option-Shift-Delete: ignore selected startup disk
C: boot from optical disc
N: Netboot
T: enter FireWire Target Disk Mode
Eject key, [left] mouse button: ejects CD or floppy disk
Mac OS X startup keys:
Shift: boot in safe mode
X: boot into Mac OS X, even if Mac OS 9 is the selected System Folder
Command-S: boot into single-user mode
Command-V: boot in verbose mode
Mac OS 9 startup keys:
Shift: boot with extensions off. If held down until the Finder loads, also stops Startup Items from loading.
There might be a few more, but I think that covers all the common ones and then some.
tooki