Do-it-yourself collectable card games! Dvorak - A Nomic Card Game: Dvorak is a card game where all the cards start out blank; players choose a theme, make up enough cards to get started, then continue to add new cards as the game progresses, provided that the other players approve of each card.
It can be played in two ways - either as a serious attempt to create a card game around a theme (a film, a book, a sport, or whatever you like), or as a ruthless free-for-all, each player pursuing their own particular aims without much regard for fairness or replayability.
As well as making your own, you can also play the decks that other people have made, as if they were standalone games - the Deck Archive contains full rules and card lists for a wide range of games, and each set of cards is easily printable. (via Invisible City Productions)