Experimental Gameplay started out as a student project at Carnegie Mellon University in 2005.
Given three basic rules (“Each game must be made in less than seven days / by only one person / be based around a central theme (e.g. gravity or swarms)”) some very interesting projects were created. Since then the website has opened up, and everyone is allowed to post their experimental games.
The experiences gained from the project are summed up in: How to Prototype a Game in Under 7 Days (Gamasutra Feature).