Another option, especially for offroad fixes, is the disc-adapter. Boone and a couple other folks make cogs that bolt onto the disc brake interface of MTB hubs, thus running your bike as a fixed gear. Alternatively, you can just drill a cog of some sort to fit.

It might be possible to do this with disc road hubs (for non-MTB spacing frames), but I don't know of any that have the 130mm or smaller spacing.