Features (incomplete list):

• Fully open-source under the GPLv2 and easy to mod
• Runs on Linux, MacOSX, Windows and Android (in development)
• Modern and fast OpenGL 3.0 and OpenGL ES 2.0 renderer
• Scriptable gameplay and user interface via AngelScript bindings (C++ style syntax)
• User Interface based on XHTML/CSS standards with support for remote content
• Support for vertex and skeletal animation
• Powerful multiplayer & e-sports 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
• And much more!

Detailed documentation and tutorials can be found on the thematic wiki pages for:

Developers | Scripters | 3D Artists | Mappers | Others