The source code for the original Sim City (the game responsible for countless hours I spent sitting in front of a computer when I was 16), have been released under GPL v3. If you ever played it (you did, right?), you had to admire all the cross-interaction of your planning decisions and certainly wondered about the underlying mechanics. Well now, you can read it first hand!
One of insightful quotes from the announcement:
The modern challenge for game programming is to deconstruct games like SimCity into reusable components for making other games!
That is a very important point, and illustrates well the unique aspect of exponential utilization opportunities of open source software. (Even if I personally believe more liberal licenses than GPL contribute towards this aspect more).