I couldn't quite get the right tension on the pedal side chain of one of my single speed bikes. I had purchased a really nice factory STS tensioner for one of my other bikes, but didn't want to wait for another one to get here. Plus they are a little pricey, about $20 plus shipping.
I went up to the local Skate Board shop and asked for the hardest wheels he had. Bought a couple and the bearings. Came home cut a groove in the center and mounted it to an old coaster brake arm I had in a box of old bike parts. It really worked out sweet, for less than half the money.