My old steel rigid MTB is now a fixed gear. Rode it to work today. Definitely on the lunatic fringe now. Hopefully my friends won't notice me not coasting and still think it's a single speed.
I don't know if I'll stick with it. It's sort of a training thing. Gotta make the most of my short 5 mile commute. Also its a winter thing. If I get good at it, I can stop wearing out my brakes and rims in the winter road muck.
The gearing is 38x15 with 26x1.5 street tires and platform pedals.