Thanks for ideas, code fragments or (other things) to the one named below (and also to the unnamed).
By ID software.
By John Carmack and others.
Games, crossing the frontier of the known.
Dawn of darkness
An excellent Quake 2 Total Conversion.
By Victor 'Vic' Luchits and others.
Qfusion is a 3D game engine based on the GPL'ed Quake II source code.
By Psychospaz and others.
Quake2maX is a 3D game engine based on the GPL'ed Quake II source code.
By Paul 'Echon' Jackson and others.
EGL is a 3D game engine based on the GPL'ed Quake II source code.
Sauerbraten Game Engine
By Wouter "Aardappel" van Oortmerssen and others.
Sauerbraten (a.k.a. Cube 2) is a free multiplayer/singleplayer first person shooter.
The aim of this game is not necessarily to produce the most features & eyecandy possible,
but rather to allow map/geometry editing to be done dynamically in-game, to create fun gameplay
and an elegant engine.
By Leonardo Boselli and others.
APOCALYX is a 3D engine based on OpenGL, OpenAL and other free libraries suitable
in particular for the development of Programming Games.
Lazarus is an assortment of entity modifications made to the Quake2 game dll.
Bullet Physics Library
Bullet Physics is a professional open source collision
detection, rigid body and soft body dynamics library.
The Ultimate Collection of Public Domain Graphics for Game Developers.
Quake 2 player models. This player models are usable as actor models in this game.
Free Music Project
Free Music for your game.