This fabulous tool is made by Jim Stein in Arizona, and when you need it, nothing else will do. It lets you remove your cassette lockring (and thus your cassette) from your rear wheel on the side of the road anywhere. It weights only 28 grams, it's only about an inch across, and you don't need a big crescent wrench or anything, just hook it to your quick release skewer and spin the cranks forwards. You can reinstall the lockring with the tool by spinning the cranks backwards when you're done.
Why might I need this, you ask? That's a good question. Being able to take your cassette off easily doesn't just mean that you can change cassettes - you can also get to the drive side spokes when the cassette is removed. If you break a spoke in the middle of nowhere, all the extra spokes in the world won't help unless you have this tool to help you remove the cassette first.