Stratagus is a free cross-platform real-time strategy gaming engine. It includes support for playing over Internet/LAN, or playing vs. computer opponents. The engine is configurable and can be used to create games with a wide-range of features specific to your needs.
Stratagus started as a clone of the realtime fantasy strategy game Warcraft II from Blizzard Entertainment, although it is used now as a game engine to build other games, sci-fi or fantasy ones.
Stratagus is developed together for Windows, Linux and Linux Maemo based devices (Nokia N900).
Games which use Stratagus:
The newest version of Stratagus can always be found here: https://launchpad.net/stratagus
Documentation on the Stratagus scripting language (Lua) can be found at http://www.lua.org/manual/5.0/
Information about creating sounds and graphics can be found in the media documentation.
Also look at the complete list of known bugs
SDL see README-SDL.txt
Guichan see guichan-copyright.txt.