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.
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.
Naval Battle is a tabletop/board game. It has a square playfield of rows and columns, each representing waterscape with islands, ports, shallow waters, reefs, etc. Each player gets an equal-sized fleet of war ships. The game goal is defeating the enemy, either by taking the royal port, destroying all ships and forts, or forcing the opponent to surrender.
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.
FreeOrion is a turn-based galaxy conquest and empire building game. It follows the 4X genre (eXplore, eXpand, eXploit, and eXterminate), inspired by Master of Orion and Europa Universalis 2, but with an independent game play. It provides a multi-player mode, quests and community-contributed story lines, complex technology upgrade paths through research or military combat, exploration, high-level strategy management, AI aliens, and lots of space ships obviously.