Mugen Infinity
Smash Apprentice
Street date of March 28th
WarioWare D.I.Y. allows players to design their own microgames, creating their own graphics, music and designing a 'cassette' for them. Players can send their creations to other D.I.Y. owners or receive other people's works. They can also be uploaded to Nintendo Wi-Fi connection for contest purposes. Microgames made available by Nintendo can be downloaded, too.
I don't really think it sinks in how awesome this game can be; what you can do with it. In these following games, they are COMPLETELY user created. The game has been out in Japan for a few months, so they've got a head start.
Keep in mind these are all 100% original and user created:
http://www.youtube.com/watch?v=K54tJDSc_T4 - Random collection (Simpsons, penguin suicide, gag reflex, ect.)
http://www.youtube.com/watch?v=vHqU4216x00 - One Piece anime games
http://www.youtube.com/watch?v=tdOR0T5JC8M - More anime related games
And there's more, uh, Japanese animation "themed" games out there, as you can imagine. But I won't post them here...