The game of Nomic, invented apparently by Peter Suber, and popularized by Douglas Hofstadter in Metamagical Themas, is the classic "play consists of changing the rules" game. Various variants exist; see Fascist, for instance. It is quite popular within a small community on the Net, having its own newsgroup and FAQ.
The idea of the Nomic family of games is to study and/or play with emergent social behaviors; the game starts (ideally) at an unstable point in rule-space, and will gradually wander towards more stable points, if any. The classic initial rules of Nomic are intended to study systems that have some of the flavor of the U.S. constitutional system, and are thus Real Big: 29 rules in about three pages of leetle beety text.
I was for a long time a player of Agora Nomic, one of the longest-running such games known to Person (I'm not playing right now, 'cause you can't do everything at once). I also, of course, created my own variant of the game.
My variant is based on my impression that more interesting parts of the rule-space can be explored starting with less baggage. So I threw together a much smaller set of rules; if the players want lots of complexity, they can vote it in. I called it "Minic" for "Miniaturized Nomic" or something like that.
Game Zero of Minic went along quite nicely (if slowly) for a few months, among a mostly closed group of friends. Here's the initial Ruleset, and here's a snapshot of the Rules as of 1996/01/15, with most hints as to the identities of the guilty removed. Shortly after this, the Player whose turn it was took too long, and the Scribe somehow never got around to kicking the world. So while the Game Zero gamestate still exists, it's unlikely to matter to anyone in the near future. Maybe when things are quieter at work (har har har!), I'll get it going again...