The Singleator mounts right up to your derailleur hanger and provides a simple, effective, and adjustable means of tensioning the excess chain slack found in vertical dropout single-speed drivetrains.

The Singleator features a triple-sealed spring, large cog capacity, chain guides, fully-adjustable chain line, floating pulley, and a stainless steel pivot bushing. The main body is forged, so it’s strong. We’ve gone with an 11-tooth pulley for smooth, quiet operation. And, it now works with both 1/8" (BMX) and 3/32" (geared bikes) chains. Beware impostors; the Singleator is the granddaddy and outperforms all others.

Surly have some good reading material about singlespeeding here.