[dwich]

I think renownedmedia is referring to the dramatic improvements made to the open source Quake 1 engine. Admittedly, id Software doesn't profit from those improvements, but it's not like they were making money off of such an old engine anyway.

[dvide]

Actually they have been known to profit from community improvements to their old engines as a result of their open sourcing. When they went into the mobile phone games market to create games like Doom RPG, there were already existing doom engine ports to work from, saving them a lot of work. I think that's kinda cool... you never know how these things might benefit you down the road.

I think Carmack talked about it briefly in one of the QuakeCon keynotes. Maybe 2006 or something.