qfusion 
qfusion is the id Tech 2 derived game engine
Features (incomplete list)
- Fully open-source under the GPLv2 and easy to mod
- Runs on Linux, OS X, Windows and Android (in development)
- Modern and fast OpenGL 3.0 and OpenGL ES 3.0 renderer, running in a dedicated program thread
- User Interface based on XHTML/CSS standards with support for remote content
- Support for vertex and skeletal animation
- Powerful multiplayer & eSports features (global stats, friend lists, IRC, TV-server, etc.)
- Ready to go FPS example gametype scripts from Warsow
- Multithreaded sound mixer design
- OpenAL support
- Hardware-accelerated Ogg Theora video playback
Extensible
- C/C++ mods (plugins) can ship new gameplay features while maintaining compatibility with the core
- UI and game mechanics scriptable with AngelScript (C++ style syntax)
- Flexible HUD scripting
- "Pure" (models, maps, textures, sounds) game content is automatically delivered to players by game servers
- Players are allowed to locally override non-"pure" game content
Notable games
License (GPLv2)
This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.
This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
See the GNU General Public License for more details.
You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.