Petzl Mini Pulley

The Petzl Mini Swing Cheek Pulley is a lightweight pulley that can handle big loads, but is small enough to fit anywhere or be carried on the harness. Suitable for use with a Prusik knot as a progress capture device, it has excellent efficiency thanks to the sheave mounted on sealed ball bearings.
 
Due to its lightweight construction, there's now excuse for not clipping one to your harness for alpine routes, glacier crossings, ski mountaineering, big multi pitch routes and for hauling duties on aid climbs.