Our lovely Beer Wrench has a titanium 15mm spanner on one side, and a bottle opener on the other. Two titanium thumb-screws attach it securely the the water bottle mounts on most bicycles (including ours).
The 15mm wrench will remove rear axle bolts, Alfine internal hub bolts, and Pitlock keys, making it perfect for removing a wheel on the road. The bottle opener opens... a beer. Or a bottle of San Pellegrino. Or any other fizzy bottled beverage you might suddenly be in need of on an extended ride.
Engraved quote from 13th century Japanese Zen Master Dogen, helpfully reminding lost riders that we're always exactly where we're supposed to be.
Weight: a paltry 35 grams.