Photoshop Ping Pong is a game where two or more players take it in turns to apply some image manipulation wizardry to an image.
Photoshoppingpong.com was created as both an Adobe training tool and a place for Photoshop users to show off their creativity and skills. Currently the site is managing dozens of games taking place in the UK and US.
This site really pushes the boundaries of user involvement in WordPress. Version 2 of the site is already in planning with much more of a community feel to it.