Originally Posted by MacG

Surly's Fixxer is the only bolt-on solution, but it is expensive. I think in the ballpark of $75. You could get a decentl entry-level hub for around $45 and have your wheel restrung onto it. Formula makes a great basic fixed/free hub. It's 120 spaced but you can get it with a wider axle to allow spacing it out to 135 or so if you need to.