Xconq is free/open source software; license: GPL.
Xconq is an engine for turn-based strategy games.
Xconq has an extensive library of prebuilt games and images.
Xconq has an easy-to-learn, yet flexible game design language.
Xconq is multi-player. Network play and computer opponents are available.
Xconq runs on many platforms, including Linux/Unix, Windows, and MacOS.
Xconq has been around a long time, and is being actively maintained and improved.