A couple of years back, Microsoft Research built a game called “Allegiance”. It was a multiplayer on-line RPG set in a rather complex space/colonies/races milieu. Pretty cool, but never really caught on. A hard-core user base evolved, but the game didn't really hit big.
I just saw that Microsoft Research has released the game source code to community. The release is governed by a Shared Source license that prohibits commercial use; that is, you can't take the source, tweak it, build a new game and sell that. But the hard-core gamers who love Allegiance can now build new servers, add extensions, whatever.
That's pretty darn cool.