I just got a single-speed bike for winter riding. The idea is that I will put on some snow tires and a fixed sprocket and just keep on going through anything short of whiteout conditions. It turns out a set of snow tires is $110, so good thing I saved a lot on the bike: the budget is still pretty tight.
($150 used via a fellow on Craigslist: about 1/4 the price of the cheapest new ones I have seen. Wheels in excellent shape and frame about 20 years old but solid.
Needs now: new rear tube (bought today, with 2 spares)
Will need soon: clean + grease chain + sprockets
Will need eventually: repack/replace bottom bracket.)