Small flat metal file
Several months ago, I was out with some friends and none of us had a bottle opener. I usually carry a Leatherman PS4, but I had forgotten to transfer it to the pants I had on. All I had was my key ring. I tried using the method shown in Marsh's instructable using the teeth of a house key to pry up the edge little by little. It worked, but it wasn't easy.
I started thinking right then about what I could put on my key ring so I wouldn't have to use that method in the future. Over the next few days I considered various options, mainly stand alone bottle openers that I could hang on the key ring. I gravitated towards the flat style because they were less bulky. Having used the bottle opener on my Leatherman many times in the past, I kind of wanted to stay away from the "hook" style - mainly because I usually had to pry it two or three times while walking it around the bottle top before the top popped off. I figured if I was going to carry a stand alone bottle opener, it should work the first time.
I stumbled onto a posting somewhere where a fellow used a metal clip similar to a carabiner to open bottles. Here's an example of the type . The 'hooked' part that the 'gate' closed against was the part that he slipped under the lip of the bottle top to pry the top off. I liked the concept but didn't care for the weight of the steel clip.
I was in an REI store and noticed a small Black Diamond Funbiner , which is an aluminum, non-loadbearing, carabineer that is about 2/3 the size of a normal mountain climbing carabineer. I noticed that the Funbiner almost had the right shape in that same area for prying off a bottle top. I figured for $3.50, it would be worth experimenting with. I filed a small flat spot on the hooked area to allow it to slip a little farther under the cap, but not so much as to affect the function of how the gate closes against that area or to create a sharp area that might wear out my pocket. See the second image. The small unpainted area is all I had to file down to get it to grab the bottle top.
I've found that with the proper technique it works rather well, and I can open a bottle on the first try about 95% of the time or better. As shown in the first image, place the hook of the carabineer under the cap. With your fingers wrapped around the neck of the bottle, use your thumb to put pressure on the carabineer upward and towards the bottle. This helps keep the hook under the cap. With your other hand, grab the back of the carabineer and pull the carabineer in the same direction your thumb is pushing (the other hand is not shown in the image because that hand is holding the camera). Typically, the gate spring will pinch the cap against the hook when it comes off so you don't have to chase the cap around the floor.
I've been using the Funbiner as my key ring now for about 9 months. I had to add a 5/8" diameter split ring to each key in order to put the keys on the Funbiner. It makes the whole key ring a little bulky, but the tradeoff is the convenience of easily being able to remove any key much more easily than when I used a 1" diameter split ring for a key ring. If I want to clip the Funbiner to my belt loop, I can do that too, but I prefer to keep things quiet in my pocket. I carry a small flashlight on the key ring. The flashlight is a handy tool to have on a key ring, but until I switched to the Funbiner, I didn't realize how much more convenient it is to be able to remove the flashlight so easily from the key ring if I need to use it for more than a few seconds.