Stein Cassette Lockring Remover
This amazing little device replaces a chain whip, spanner and chunky cassette tool. It's designed for cycle tourists who need to be able to remove their cassette so they can replace broken spokes on the rear wheel (it's amazing how many people carry spare spokes but not the tool to fit them!).
It's not something you'd want to use all the time, but then again, if you're breaking spokes all the time, you probably just need a stronger wheel.