I often make my own beer at a local brew-it-yourself taproom (props to The Brew Kettle). The bottles we use are 22-ounces, so drinking one is almost like drinking two. Often times I'll end up drinking more than I wanted or drinking none at all (oh, the horror).
Stumbled across the Hermetus Bottle Opener and Sealer while looking for a Father's Day gift for my dad. Bought one for him, a couple guys in the brew group, and myself. To create an airtight seal simply slip it over the top of the bottle. It works perfectly.
Drank half a bottle one night then sealed it and put it in the fridge. Drank remaining half the second night, and it tasted the same and still had a nice head on it. I love the simplicity of the design!