I am currently working on a flash game prototype where the entire world will be created and designed by players. Most of the core concepts are in the game, but I only have my crappy attempts at pixel art:
As you can see from the screen above the game is made up a grid of 24x24 pixel blocks. This game was inspired by other great indie games like Proteus, Knytt, and Minecraft so it's really important to have a unique look, atmosphere, and world.
I do have a cool time of day effect working:
As you can see from the screenshots the bottom bar tells you your current location, in the bottom right the user that created the level is displayed along with reddit style voting buttons. Levels with higher scores will be harder to re-design, but ultimately this is really just one big experiment.
The game requires that users login and tracks their experience (which unlocks abilities like double-jump), credits, and world location. Initially players are dropped into the same 'home' location screen and are free to explore - if they visit a location that does not have a level, they are put into a design mode that lets them draw a new level and add it to the world.
I would love to collaborate with someone on this project. Let me know if you are interested and feel free to leave feedback below.