Looking for a way to liven up your commute? Are you a masochist? Do you have money burning a hole in your pocket? If you answered all these questions "yes", then a single speed fixed gear might be the bike for you!
A single-speed fixed gear (also known as a "fixie") is a bike with most of the useful parts taken off: no front or rear derailer, no freewheel or cassette on the back, only one chainring on the front, typically only a front brake (or no brakes at all if you're the sort who likes the inside of hospitals), no drop bars, etc. The key idea here is "minimal".
These are show bikes, in the "ooo look at the pretty bike" sense, but also the "check out how hard-core I am" sense. Not being able to coast or shift gears means you'll be really grinding up hills and pedalling like a maniac when you go back down. But without all the cables and
derailers it weighs a lot less, so it's not such a big deal.
I rode my fixie in to work today, and it was a lot more fun than I remembered. While it's possible to go fast on these, that's not really the point. The point is going in *style*. Unless you're a bike courier, and then the point is going fast in style. Other cyclists will almost certainly realize you're never coasting or changing cadence; the lack of a rear derailer and clean line of the chain is eye-catching too. Riding one requires enough new skills that it's almost like learning to ride all over again, and there are tricks you can do on a fixie that are impossible on any other bike, like downhill track stands, or the Sheldon Brown Fixed-Gear Dismount. If you're insane you can do all sorts of circus tricks on a fixie.
Interested? Check out Sheldon Brown's Fixed-Gear Page for more information!