Park Tool GSC-1 Gear Cleaning BrushPark GSC-1 Gear Cleaning Brush - Bike Specific Cleaning Tool
If you've got a cruddy, grungy, disgusting bunch of gears, then this thing, coupled with some degreaser, is one of the best ways to go. Unless you have a parts washer of course! The teeth on the back of the handle break up and pull out the dried-on gunk pretty well, but you do have to wipe it off pretty often. The brush is okay at best. It's good for light scrubbing, but it gets mangled and messed up pretty quickly.
If your short on time but want to get alot of grime out this tool is great. No tool is better for getting rid of muck out of your cassete plus rear mech.