Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
Park your gum there.
And Google "FireWire devices" to provide you fuel for your imagination.
 
Probably the same thing you did with the other one. ;-)

Hard Drive (chain of many?) if the spare port supports it (you didn't say which Mac you have), go with FW800
Optical drive
Original iSight (the barrel one, with auto focus)
Some video cameras
Audio a/d, d/a, midi
Original iPod models sync

That's about all I can think of, at the moment.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.