Layer Tennis

Designer geeks rejoice, you need no physical activity skills, for your sport has come. Layer tennis is a site where designers, artists and illustrators face-off in a game that uses Photoshop as its court. The players swap images and score points with creativity.

From their site:
Two artists (or two small teams of artists) will swap a file back and forth in real-time, adding to and embellishing the work. Each artist gets fifteen minutes to complete a “volley” and then we post that to the site. A third participant, a writer, provides play-by-play commentary on the action, as it happens. The matches last for ten volleys and when it’s complete, everyone with an opinion sounds off in the Forums and we declare a winner.