TIGSource's Assemblee is a two-part game making competition: The first part (which is running now, through November 28th) is a call for artists and musicians to create assets: tiles, characters, UI elements, fonts, sound effects, whatever. These go into a large communal pool, which will be used freely, mix and match style by programmers and designers to make games, come the second part - all the art used for these games come out of that pool. You can submit whatever you like, so long as it's made for the competition.
Read more here.
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Link is broken, I think... I already saw a competiton like this, but the programmers made a game first, wich only one would be chosen, so artists can create graphics for it. When all graphics were done, only one is chosen.