DMR STS Chain Tensioner
Designed for riders who require tough components. The Stainless steel DMR STS Tension Seeker is fitted with a double-sealed bearing pulley to ensure its toughness. Bolt this on to your derailleur hanger to help take up the slack on your single speed machine.
I have used and installed this tensioner on a variety of bikes and this is the next best thing to horizontal dropouts. once you set the tensioner where you want it, it stays put and you don't have to worry about any spring mechanism that you can't adjust. The only downside of this product is the lack of spare parts that DMR offers; once you break that little black plastic piece you have to shell out enough money for a whole new tensioner.