Ever since The Sims was released in 2000 (and I guess you could include SimCity before that), user-created content was only something hardcore modders could grasp. But with the original Sims game, along came tools of the Transmogrifier, Homecrafter, and a wide range of other programs to help create user-content. The game exploded in popularity, thus introducing 2 sequels.
The thing is…it appears that there are no other games that has based themselves on this idea….until recently. LittleBigPlanet for PS3 will have a huge impact with user-created content, and more games are beginning to take on this concept. I myself look forward to LBP as it’s receiving a lot of praise.
Check out the article below to further reading on this subject.