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/
Information about creating sounds and graphics can be found in the media documentation.
Information about Stratagus Lua scripting can be found in the scripting documentation.
Information about metaserver protocol is in metaserver_protocol.txt.
Also look at the complete list of known bugs