When it comes to things that live on your keychain, smaller is pretty much always better. So a bottle opener that weighs basically nothing, and as is as small as your fingernail, sounds like a pretty great deal.
PiCO is a wannabe miniature titanium bottle opener currently crowdfunding on Kickstarter. The idea is to distill the essence of a bottle opener down to something as small as possible, that still just about works. As such, it doesn't really have much of a lever, forcing you to rely instead on the keyring (and brute force). I think it still looks better than that opening-bottles-with-your-teeth trick, though.
As always, the usual crowdfunding buyer-beware warning applies; though, with something relatively simple like a single piece of metal, you'd hope that the logistics won't be too complicated. $US11 will (hopefully) score you a tiny bottle-opener sometime in June this year, leaving you money left open for trial six-packs of beer. For, uh, science, of course. [Kickstarter]