Doom 3 recently went open source, as is usual for their games when they release a new game engine.
http://en.wikipedia.org/wiki/List_of_commercial_video_games_released_as_freeware
Yep, but the game data isn't free, so you still have to pay for it to play it.
indeed, going "Open source" gives a game a greater chance at being able to evolve with the times.
Community made content will come in time, I'm betting. It generally takes a bit, but I'm sure it will come. Assuming of course it has a healthy community, which I don't know about. I never really got into its community, to be honest.
open sourcing is about more than content it can allow someone to truly change a game