A shareware game called begin was released in 1984: a tactical starship game with a good AI and fair rules (the computer had no advantage) I've hacked assembly code to make it a multiplayer game, so that folk can challenge each other rather than the AI.
JustEmpire and TEmpire (Tiny Empire) are two Empire-derivative games based (very!) loosely on Walter Bright's original Empire concept (classicempire.com), using JustBasic (justbasic.com) and Borland Turbo C/C++ (bdn.borland.com/museum). Others possible.
Signus: the Artefact Wars is a highly-playable turn-based strategy game with 19 long missions, lots of precisely designed units and very good AI.
Maintained fork is at https://github.com/signus-game/signus
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Forgotten Destruction is a free real-time strategy game developed using the Simple DirectMedia Layer. It focuses on the conflict between an evil shadowy empire and human race with their nearby allies.
Tourk is a framework for developing programming games, where two (or more) people write game playing code (like Deep Blue) and a controller puts the two against each other.
A game based on one of Microprose`s best sellers, Colonization. This is an open project that carries the idea to create a strategy game with elements of adventure that`s set in the world during the colonization era (1492-1830).