I made this choker a while ago, and I really like it.
It's very simple, but I'll make this a how-to anyway. I'm amazed at how much I wear it.
If I were to do this again, I'd make the hole slightly off center, and use a cap without text on it, just a logo. Though I do think that the fact that I used a cap that wasn't pretty makes the necklace more interesting. I like the character it has: it really is an adornment that is just a piece of junk.
Select your bottle cap.
Mine is rather junk-y, because it's the only one I had on hand at the time, and I was bored and hammering nails through things. But you get to pick any bottle cap you want, so you'll probably want one with bright colors, a cool design, or from your favorite beverage company.
Carefully drive a nail through your bottle cap with a hammer.
You can make your hole in the center like I did, or put it off to the side, so the bottle cap hangs nicely like a pendant.
String some leather through the hole, and tie it in a knot.