Doom 3 might technically qualify, the source code was released under the GPL years back. The caveat was that you needed the creative assets to play it (although I'm pretty sure it would happily run with custom maps).
I especially like this model. You get the source but the people who worked on the game still get paid.
Romero spoke about this a while back. Several members of the team at ID, Romero included, got started on the Apple ][ computers. Those machines were completely open, software driven everything! I keep one as a reminder of my own roots, and the machine is still fun! (Romero keeps his too, as do many from that time period, but I digress...)
The author behind Beagle Brothers software and educational materials was a huge impact on many of us, and ID. They released their code so that others may create and so that their games live on for the people who enjoy playing them. Open matters, and that's why one can play doom on almost everything these days, as one example.
It's a great model.
What I've enjoyed is seeing the ID team watch what others do, and share in all the fun from a secure position having been well paid, no worries beyond that.
And frankly, they can STILL sell those creative assets. People would still buy them.
I especially like this model. You get the source but the people who worked on the game still get paid.