Yet for some reason extremely few games have followed id's example. Besides Duke 3d, what other games ever saw source release?
SoftDorothy: https://github.com/softdorothy
I wish there were more.
Edit:
Abuse: http://abuse.zoy.org/
It was so rare back in the day that I vividly remember games that did actually go open source.
we played hours and hours every week of enemy territory in the office in 2007-2009
... on Linux (and the occasional Windows and Mac in the same server) no less!