Problem Solvers 6 Bolt Disc Cog
This is kind of a cool product. The holes line up with 6 of the holes on a standard ISO disc brake rotor mount. So by flipping your rear wheel around and bolting this on the disc side, you have a fixed gear, for not a lot of money.
Problem Solvers used to sell a carrier for these, to mount them on a cassette body. We don't have any of those, and would recommend using a Surly cog instead.