Not exactly the discovery of a new fundamental particle but far more relevant to having fun, the Petzl Reversino is a tub-style belay/rappel device with a self-locking feature that makes it convenient to belay from the anchor.
The 2oz Reversino works with ropes 7.5 to 8.2mm, perfect for double ropes and smaller single ropes commonly used for glacier travel rigging. For 8-9mm double ropes and 10-11mm single ropes you can step up to its big brother the Reverso. Both are similar to, and lighter than, the Black Diamond ATC Guide, although the BD device uses its extra material to good effect and is really smooth.
I use the 3.6oz ATC Guide for general belaying duty and the 1oz DMM Bugette for low-angle glacier work. I picked up a Reversino for high-angle alpine routes where some pitches might be better belayed from the anchor.