Here are a few
c -- boots from an optical disk like a gray system install/restore disk or a full retail install disk
shift -- boots into "Safe Mode" with most extensions turn off. SM also does some disk checking, so can take a while before you get a desktop.
option -- opens the Startup manager where all bootable volumes appear as icons. This also takes a while to come up
t -- boots into FireWire Target disk mode where, if you are connected to another FireWire-equipped Mac. allows the otehr Mac to see yo uG5 hard drive as a mounted external drive.
x -- boots to an available OSX volume (more useful on older PowerMacs hat can boot either OS9 or OSX from different partitions)
NOTE 1: these are lower case letters. Don't use the COMMAND or CONTROl keys to modify them
NOTE 2: Some of these may not work from a non-Apple keyboard.