I've been really digging Coudal Partners' Layer Tennis lately. It started just last Friday, but the first two matches have been excellent.

Basically, one player starts by delivering a prepared-a-day-in-advance picture, and then the second player gets 15 minutes to respond by modifying the picture, changing the color palette, moving objects around, incorporating elements into his own creations, adding new text or images, etc. When the second player is satisfied, he delivers the volley and the first player gets to respond. There are a total of 10 volleys including the serve and the parting shot. Also, each player gets to use the Photoshop file with all the layers and such from the previous volley so it's very easy to manipulate elements losslessly.

There's also a moderator who entertains the visitors while they're waiting for the next volley. John Gruber of Daring Fireball fame moderated the initial game.

The first match, Shaun Inman vs. Kevin Cornell, was very heavily design-oriented and incorporated some very cool typographic elements as well as a humorous, poking-fun, narrative. Yesterday's match was completely different, concentrating on outlandish imagery and symbolism and bizarre themes. It was a complete about-face from the first match, but it was still incredibly fun to watch.