Fixed-Gear Bicycle
Pros:
They're really light, relatively easy to repair, and in a flat urban environment can go a little faster for the effort put in. They also supposedly promote more efficient cycling.
Cons:
You can't coast with them, so they can be really dangerous to use if you don't know what you're doing, particularly in traffic. And they make hills pretty much hell.
Also, a lot of stupid as hell hipsters only get ones which have no brakes because they're more \\"pure\\". And since most hipsters aren't experienced bike messengers, being anywhere near a hipster on one of these can be hazardous to your health (or your car's health).