I don't know how well known this stuff is, but my friends and i used to be pretty loyal to HaffenReffer Private Stock. (AKA Headwreckers) a 6pack for 5 bucks.
These things kick my ass all over the place. And under each cap is a playing card number&symbol (10/spades, queen/hearts, etc) Someone once told my best friend and I that it's nearly impossible to do, but If you collect a full deck in bottle caps, you get a free year supply or something. Hehe, my friend and I, being oh so young and impressionable at the time, made it our goal for one summer to collect em all. By September, we had 2 full decks... but no year supply, apparently that ended up being just a myth. .. lol