Firewire target disk mode
From Applepedia
Firewire target disk mode is a feature of the NewWorld ROMs on the Macs with built-in Firewire. If you hold down the "t" key on your keyboard when you boot up your Mac, it will enter this mode, where the computer emulates a Firewire disk. This means you can plug your Mac into any other computer (even a PC, assuming it has software to read an HFS-formatted disk), and as long as it has support for bog-standard Firewire Mass Storage devices, it'll show up.
This can be very useful if you've broken your System install, or you're having disk troubles.
[edit]
