The coolest thing I've encountered all week:
A roleplaying game/collaborative-worldbuilding system called Lexicon, "in honor of its inspiration, Milorad Pavic's Dictionary of the Khazars." (The game was created by Neel Krishnaswami.) From the introductory description:
The basic idea is that each player takes on the role of a scholar, from before scholarly pursuits became professionalized (or possibly after they ceased to be). You are cranky, opinionated, prejudiced and eccentric. You are also collaborating with a number of your peers—the other players—on the construction of an encyclopedia describing some historical period (possibly of a fantastic world).
On each turn, everyone writes an encyclopedia entry beginning with the next letter of the alphabet. The entries have to refer to other entries written by other people. I'm particularly enamored of rule 3:
Despite the fact that your peers are self-important, narrow-minded dunderheads, they are honest scholars. No matter how strained their interpretations are, their facts are accurate as historical research can make them. So if you cite an entry, you have to treat its factual content as true! (Though you can argue vociferously with the interpretation and introduce new facts that shade the interpretation.)
I so want to play this game. I can't possibly afford the time to do so, but I really want to. If someone sets one up, please let me know, and I'll see what I can do. See the Lexicon rules page for the full (yet brief) rules, and some discussion of same (in the comments that follow the rules and the trackback links). (As one of the comments notes, this game can be seen as a variant of the letter game; I'm amused by the pun on letter.)
Note that a wiki is an ideal place to play this.
Thanks much to Greg I. for pointing me to this! If you want to see a completed example, you can look at the Paranoia Lexicon game that Greg participated in, which apparently was used as source material in the new Paranoia XP edition of the classic roleplaying game. The new edition is published by a British company called Mongoose Publishing; they're also the new publishers of the Babylon 5 RPG, and of a Jeremiah RPG (presumably based on the comic book and the JMS TV series) which I'm half-tempted to get just to see whether the GM section explains the parts of the backstory that the series never got to (at least not before I gave up on it).
But I digress. For another example of Lexicon, this one inspired by Borges, see the Telephone Directory of Uqbar; unfortunately for me, it's in Italian, so I can't read it.