Petzl Stop

Works best with flexible rope, the Stop locks down on the rope if your hand comes off the lever. Safe device with a long history of use in Europe for caving.

I used the Stop in Europe to descend various caves over the course of a month. It is a good device, and allows for a free hand to clip into a belay station on rappel. This is very nice since the ropes in Europe are generally re-anchored multiple times on a rappel.  

It is also fairly compact and lightweight compared to a stainless rappel rack. Unfortunately, I don't get to use it much, as most U.S. caving ropes are stiff and don't work well in the device.  

Tip: Make sure to close the device for storage or while caving so you don't accidentally bend the cover plate.