Stein Mini Cassette Lockring Driver
When you need it, nothing else will do. The Stein Mini Cassette Lockring Driver lets you remove your cassette lockring (and thus your cassette) from your rear wheel. It weights only 28 grams, it's only about an inch across, and you don't need a big crescent wrench: just hook it to your quick release skewer and spin the cranks forward. You can reinstall the lockring with the tool by spinning the cranks backward when you're done.
Why might I need this, you ask? 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.
This fabulous tool is made by Jim Stein (JA Stein) in Arizona.