[[Datei:Singlelator.jpg|center|Surly Singleator mit Federspannung]]
These are like partial derailers, with a single spring- loaded pulley. They attach to the derailer hanger of your dropout, or to an adapter claw. Like many purpose- built singlespeed parts, they are often rather expensive considering how simple they are. If you plan to use different gear ratios, for instance with a flip- flop hub, a spring-loaded tensioner is the best choice.
You can also use your old rear derailer as a chain tensioner, but it is heavier than you need, introduces more friction, and detracts from the looks of the bike. You need to adjust the high-gear limit stop for the correct chainline.