OpenRA is a real time strategy gaming engine, which reimplements Command & Conquer: Red Alert and Tiberian Dawn mods, as well as Dune 2000. They're not exact copies, but allow modern cross-platform gameplay with new features and graphical improvements.
An Open Source 90s Pixelart Science-Fiction Real Time Strategy game with Multiplayer (LAN and internet) support, competent skirmish AI as well as an integrated map editor. All the assets are licensed under Open Source compliant terms.
Pioneers is a PC game version of the board game The Settlers of Catan. It can be played against the computer, or over the internet by finding up to 8 players per gaming metaserver. Custom layouts can be defined.
The Battle for Wesnoth is a fantasy and turn-based tactical strategy game. It features single-player campaigns and online multiplayer combat. Players build an army by controlling villages, quiring gold and fighting enemies for experience and growing new abilities. There different unit types with varying abilities, six major factions, weapons and spells. Maps can be customized, and scripting for new factions and campaigns is simple.
FreeCiv is an empire-building strategy game inspired by the history of human civilization. The game commences in prehistory and your mission is to lead your tribe from the Stone Age to the Space Age. It's generally comparable with Civilization I & II, allows up to 126 players, is based on artificial intelligence (AI) computer-controlled players, allows network games, comes with pre-made maps and scenarios, extendable trough mod packs, and is fully internationalized.
FreeCol is based on the classic Colonization and Civilization game. It's a turn-based strategy game about colonizing a new continent, nation development, technical and financial progress. FreeCol was originally implemented in C/C++, but is now Java-based.