On December 21, 2001, the source code for Quake II's engine was released under the GNU Public License.

Here's John Carmack's .plan update on the subject:

Name: John Carmack 
Email: johnc@idsoftware.com 
Description: Programmer 
Project: 
Last Updated: 12/21/2001 19:07:50 (Central Standard Time) 
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December 21, 2001 
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The Quake 2 source code is now available for download, licensed under the GPL. 

ftp://ftp.idsoftware.com/idstuff/source/quake2.zip 

As with previous source code releases, the game data remains under the 
original copyright and license, and cannot be freely distributed. If you 
create a true total conversion, you can give (or sell) a complete package 
away, as long as you abide by the GPL source code license. If your projects 
use the original Quake 2 media, the media must come from a normal, purchased 
copy of the game. 

I'm sure I will catch some flack about increased cheating after the source 
release, but there are plenty of Q2 cheats already out there, so you are 
already in the position of having to trust the other players to a degree. The 
problem is really only solvable by relying on the community to police itself, 
because it is a fundamentally unwinnable technical battle to make a completely 
cheat proof game of this type. Play with your friends.