Problem Solvers Chain Tensioner and Derailleur Hanger
This little device will stop your rear wheel moving forward when you stomp on the pedals. It will also give you a derailleur hanger to bolt a derailleur onto. That makes it very useful for converting track frames into geared city bikes.
Will take either a 10mm solid axle, or a quick-release skewer.
For frames with track ends (also called rearward-facing horizontal dropouts).