"Counter Strike", still the most played FPS today, was develop as a "half-life" video game mod by two college students...
This demonstrate the incredible value and creativity that amateurs can produce if they are given the right tools (often called a SDK "Software Development Kit" which come with some games).
Not surprising that "Valve Software's Doug Lombardi has stated his strong belief that user created content is a very important part of games in the near future"
See the slashdot news and article.
I would add that in the 3D world it goes now far beyond videogames.
"Second Life" popularity come in good part of the objects the users can create (and even sell). Also Google Earth and Sketchup enable individuals to easily become 3D architects who then add a new "dimension" to Google Earth (see 3D Warehouse).