Benjamin Franklin (and posters in 1 out of every 3 partying nerds' dorm rooms) said that beer is proof that God loves us and wants us to be happy. While the truth of that statement is up for debate, it's clear that beer bottles are proof that people who make beer sort of hate us and want us to be frustrated. What to do when the fates align to put you in the same room as a drinkable beer but cruelly leave you sans opener? Think of it as a MacGyvering opportunity.
My first line of defense when I'm confronted with an unopenable beer is to search the room for someone who will open the beer for me, because I usually tend to spill beer on myself or others when opening a bottle that isn't twist off. If I'm at a party with mostly people I know, I'll ask the most manually adept person— the woman who fixes bikes, for example— to do the dirty work for me. It's for everyone's own good.
But if that's out of the question, like let's say for some reason you're at a party where no one likes you or you're trying to impress people by demonstrating your bottle wrangling skills, (or if you're at home alone with only an unopened beer to keep you company), you'll want to look for items that fit into one of three categories: hard, smallish flat items that can be used as a lever, furniture or large stable items, items that can be used to uncrimp the bottlecap, and human body parts.
Lever-like items that can be used as a beer bottle opener include wooden spoons, silverware handles, the edges of pots or pans, etc. Just make sure to get an edge of the item under the lip of the cap, hold the bottle steady, and force the edge of the cap off of the bottle. If you're using silverware, grip the bottle tightly and use your finger closest to the top of the bottle as the lever's fulcrum. The edge of a key can be used as well, as can a staple remover, a belt buckle, the edge of another bottle (not a beer bottle. Even though this is possible, it can end in disaster),
If you can't find a handheld bottle opener type implement, look to larger objects, like furniture. The edge of a counter can provide the stability needed as you force the corner of the bottle up. The back of a chair can do something similar (just be careful that you don't leave a mark on a chair that isn't yours). In some cases, I've found success using kitchen drawer handles. The trick is to hold the bottle solidly and provide a sharp jolt of force at an angle to detach it from the bottle. I've also seen it done with a hockey stick, the coin return slot of a vending machine, a pay phone, the inside of a door jamb (the part where the door latches), the edge of a radiator, and a stable metal stepstool.
For those of us who fear their own He-Man strength, there's a more conservative and slow way to open a bottle: uncrimp the cap so you can gently lift it off yourself. The tines of a fork can be used to bend the cap so it's flatter, as can a nail clipper. A pen can also be used to slowly uncrimp the cap.
As a last resort, befriend someone who can use their teeth or eye sockets to open beer bottles and just take them everywhere with you. We've all got That Friend, right?
Remember, these techniques take a steady hand and precision, which is why before deploying them in public, you should practice them at home. If you practice opening beer hard enough, you may never have to leave the house again.