SPECS:
Currently my Sport Utility Bike -- with slightly shortened mtb bar, old Shimano Tourney sidepull road brake, and my first fixed rear wheel. Seat is a San Marco road, Gearing is 39x18.
Paint is mostly black, with gold where it used to be red; accents the black frame nicely. The top tube, near the seat tube junction, bears the words "NO GEARS" also in gold, with a skull and crossed bones between them. Nice touch, I know... ;)
* So why build such a bike? Simple: It's fun, it's practical, and it cuts down use of my car!
- Why a mountainbike bar? Somewhat more upright than road bars, but slightly more agressive than the roadster bars this bike had on it before. As this is my utility bike -- errands, store runs, and everything around town, or the next town, etc. since I hate using my car and wasting gas, it doesn't need road bars.
- Why low gearing? Around town there are plenty of traffic lights, and many of them back up. The 39x18 allows me to accelerate quickly from a stop, and beat the cars at the light. Plus it makes hills easier esp. when carrying a pack of stuff. Finally, though such a gear accelerates quickly, it spins out pretty fast; it picks up speed quick, but it's top speed is relatively low because you just spin after a while. So it helps keep me from going *too* fast in traffic where a sudden stop may be necessary given the morons that drive around here in NJ.
Low gearing is also good because in conjunctin with thge 'cross tires, it enables the bike to make occaisional offroad jaunts through deer paths and wooded trails, dirt roads, etc. - Why a BELL? Aren't they for kids' bikes and clowns? Hardly. Yeah, a bell would look stupid on my road bike. But a fixed gear built for the street is already an unconventional thing, so a tiny bell won't be ruining any "classy" vibes, and besides, it beats yelling "outa' the @#$^ing street!" at every intersection when people step in front of you without looking. I have mounted the bell to the bar stem right now, nice and out of the way. A fixed gear should have a bell, just because it usually runs quieter than a geared or coasting bike, and people cannot often hear you coming unil you are right on top of them!
- Why single sided SPD pedals? Because sometimes I just want to cruise to the corner for a cup a' joe or to pick somethin up at the store, it's only a few miles or it's at the spur-of-the-moment, and I don't feel like throwing on my cycling shoes. Also in sketchy areas, be they an uneven dirt trail or a busy parking lot, you can keep your dominant foot unclipped while you pedal, riding on the platoform side, ready to put it down if you have to stop suddenly.
*- The origin: After a December 26 '04 chance off-road venture on one of my earlier road fixies, I decided to build up a fixed gear meant for offroad use. The frame is an old Centurion, some of those old Japanese frames are sweet and this is no exception. Great head tube lugs, frame is in good shape, and it was cheap; $15 or $20 at a yard sale -- don't recall exactly. It had sweet looking thin, 165mm SR Apex cranks dated '76, making it mighty old! But it rides great. In it's current build, this was to be the winter/around town ride but also doubles as a offroad fixie.
*- The build-up: On the evening of 3-31-05, I added mini fenders and brought the gearing up from 36x18, which was strictly offroad though passable on the local streets, to a more managable medium of 39x18 which allowed around town riding as well as the occaisional offroad jaunt. On 4-02-05 I swapped out the roadster bars for a mountainbike bar, shortened about an inch on either end, and began applying gold paint in place of the red accents. The following day I switched out the old Brooks seat for a San Marco which was a little more "roadie" and yet comfortable for local riding due to having somewhat more padding than a "racy" road seat. It had been the stock seat on my Lemond road bike, but was replaced by a thinner one because at faster speeds it actually wasn't that comfortable, but for this bike's mission it is just the right balance betwixt comfort and speed, kick-@ss and sore @ss.
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The once all-black frame sported first red accents at the lugs, now gold, courtesy of my poor painting skills.
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This frame has seen earlier fixed incarnations; below it's former description:
My Bianchi was my first fixed gear, but this was the one i used the longest. I rode it regularly to Madison, NJ a nice 20 mile round trip, when i ran a 48x18 and cowhorn bars, as seen here 
Many a day, it'd be parked outside the now defunct Sweet Dreams Cafe, home of the best coffee in the world, and live music, in downtown Madison, NJ, right on Lincoln Place opposite the train station.
The OLD SKOOL CENTURION: Around June this bike was a repro of a turn of the century track racer, 165mm cranks, 52 tooth gearing in the front [with an 18 tooth rear cog to cope with hills] inverted cruiser bars and custom red and blue accents. 
On July 27, facing the loss of my automobile, I built it up as a utilitarian fixed gear, running 39x16 with flat mtb bars and a front handbrake and rack. Late July '04, I deciding to build up a sport utility bike to use for around town and spare my Panny the drudgery of errands, reserving it for longer rides, i initially used my Bianchi frame. The Bianchi was lighter and handled sweet -- plus it was my first fixie. Unfortunately, it also had a good deal of beat-up wear and tear and the frame makes a rattling sound indicative of some internal rust. The Centurion doesn't have this problem. Plus, the fender eyelets on the Centurion are beefy enough to permit mounting a rack. i took the flat bars and brakes off the Bianchi and swapped 'em to the Centurion. It was sweet. I painted over some of the outlandish "old skool" touchups, except for the headtube and seatstay cluster, the frame tubing is all black again. Looks great with the Road VX cranks because of their sweet cut-out bolt-arms.
Then on August 8, I fitted it with drop bars. Until i can buy a 2nd fixed gear rear wheel it must share with the Panasonic, but it is my 'round town bike -- and would do for longer rides too, with the easy [39x16] gearing and addition of the drop bars!
I'm using a Planet bike red light now for the back, but this thing originally had another one... A great light that was indestructable. i still have it but need to find an appropriate mount.
Personally I like riding brakeless but if you ride in traffic on hills a little "insurance" doesn't hurt, drivers are dumb and the dumber they are the more enormous the tanks they putt around in... That little taillight has a funny history. I got it as a gift when I graduated high school, that Christmas if I recollect, with some other stuff. I had just got back into bike riding. Once university started I rode the ten [technically 9.74] miles each way to classes whenever I could, even in the rain. First on my TREK mountainbike, then on a used Puch Alpina road bike with bar end shifters -- my first bike with presta valves! I've had that little light since 1996 and it's survived like three bikes and I don't know how many crashes -- I don't count the minor ones anymore. All I know is that little light keeps going. Wish I remembered the brand, I'd recommend it!
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[previous state of this bike: Cowhorn bars and front brake, 48x16 gearing, then 48x18. Thent he frame was mothballed and parts used for my Blue Bianchi, and later my Trek 1000 fixed... AS OF 2004 the Centurion is in mothballs awaiting the $ to replace a bottom bracket with chronicly loose spindles that causes the left crank to shake like it has parkinsons. FOLLOWING are the specs and info on this bike. My Centurion fixed gear is currently running 48x18. [the 48-tooth 'ring put on 11/22/03, and a week later I got the 18 tooth rear cog, makes hills less slow]. It is really fun to ride. If you haven't ridden a fixed gear, try it. Really.
Fixed gears are sort of an underground thing, so most bike shops will have to order parts for you. But it is possible to build up any decent road bike to be a fixed gear, though the vertical rear dropouts on new road bikes make moving the wheel back to tighten the chain hard, and keeping the chain tight is very important. One of the favorite rides is through the great swamp [NJ] coming in through Meyersville and going through to Green Village, then on to Madison. The Fixed gear works good here because the swamp is flat, but after I clear the swamp proper, there are hills of a sort. The 48 makes short work of what the 52 was harder at. One of the coolest things is coming back from Chatham and climbing up the big hill, and passing that point where the outside of the leg hurts -- it actually starts to feel good, and moments later I'm catching a buzz from pedaling!
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Top Reasons a Fixed Gear is Cool: Because it rides up hills easier. Because the simple drivetrain and smooth flowing chainline looks cool when it's leaning against a wall. Because you can go backwards! Because it was the original bike.
Pic of Centurion fixed gear above.
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