The Tr!ckstuff Exzentriker is a great product from Germany turns your outboard bearing bottom bracket into and eccentric bottom bracket.  Now you can turn any bike with an English bottom bracket shell (mountain, road, cross, etc) into a fixed or single speed.  The best part is if you have a nice wheel set that you want to use you still can.  I know you are thinking I can get a White Industries Eno hub for more than $100 less, but if you factor in the cost of a new rim, spokes, and labor the Exzentriker makes sense.  And you can still use your wheels.  

Works with cranksets that have a 24mm axle.  (Shimano, FSA, RaceFace, ETC)  Inner chain rings on triples may need to be removed.  English threading only.
The bottom bracket axle eccentrically rotates allowing for chain tension adjustment on a frame with vertical dropouts.