This Black & Decker "Auto Wrench," a self-adjusting
monkey crescent wrench that automatically sizes itself to the bolt you're on, has been around for a while, but I'd forgotten about it and thought perhaps y'all might have, too. And bonus: it's been out long enough to garner some user reviews, which are almost universally positive. You can get it at Amazon for $29. Two AA batteries will power the clamping jaws for about 650 cycles.
Catalog Page [Amazon via Coolest-Gadgets]