I use the FW800 port only to run a WD My Book for Time Machine... probably could do it with USB, but both iMac and the HDD have FW so why not? Otherwise, I don't have anything else that uses FW.
I use mine for my iSight, Target Disk Mode, and it makes a convenient iPod Charger (For the older iPods that can still charge through it)
It is also great for time machine hd's.