Paul Components Melvin
The Melvin bolts to the frame where the derailleur would normally go and allows the use of a single speed rear wheel on bikes with vertical dropouts.
Vertical dropouts have the slot for the hub axle going up and down. This keeps the wheel at a certain distance from the cranks, but nine times out of ten though it doesn't put the wheel in the right spot for a single speed wheel and the chain is too tight or too loose. The Melvin takes care of the chain tension and lets you run a single speed wheel on almost any frame with a derailleur hanger.
This is the old version with no sealed pulley, this is the reason for the lower the usual price. Only one black left in stock.
Thanks Andreas for the pictures!