The good news: most of Apple's professional-level computers -- the Power Mac G5 and the aluminum PowerBook G4 product lines -- feature built-in FireWire 800 ports. The bad news: most people I know don't yet own any FireWire 800 peripherals. They're still making do with the older, slower FireWire 400 drives and scanners they bought way back when. That means the FireWire 800 ports on their Power Mac or PowerBook are going to waste.
You could spend beaucoup bucks to upgrade your peripherals if you've got the cash and the inclination. Or you could just pony up $15 for Sonnet Technologies' FireWire 800 Adapter (4 Mice) and use your FireWire 400 peripherals with your FireWire 800 port. One end of the adapter plugs into the FireWire 800 port; the other provides a standard FireWire 400 connection. Your FireWire 400 hard drive won't suddenly acquire FireWire 800 speed, but at least you'll be able to connect it.
If you've got a desktop Mac, you might find that the Sonnet adapter protrudes a bit too much, or that your FireWire cables are putting a lot of pressure on the adapter (and thus the FireWire 800 port). If that's the case, WiebeTech's $20 FireWire 9-6 cable (4 Mice) provides the solution. Like Sonnet's adapter, the WiebeTech offering lets you use FireWire 400 devices with your FireWire 800 ports. The difference is that the FireWire 9-6 is a stand-alone cable -- one end plugs into your FireWire 800 port, the other plugs into a FireWire 400 peripheral. It's available in 1.5- and 3-foot lengths. Either of these inexpensive adapters can quickly increase your Mac's connectivity.