FWIW the ET Legacy[0] project remains active, and exists because the Wolfenstein: Enemy Territory code was GPL'd.
I don't know if that's a good example of the 'best aspects of the GPL' that Carmack was talking about, but it's at least certain that all of the work contributed to ET Legacy is work that had the potential to be closed-sourced, but wasn't.
It would be really interesting to see the lineage of open-source projects based on the GPL-licensed id games. The family tree at wikipedia[1] isn't complete (doesn't show ET Legacy, for example), and doesn't differentiate by license.
