A full open source reimplementation of Zero Hour. Impressively far along and intends to be cross-platform in the future but Windows only for now.

Open source tower defense!

A Victoria2 recreation as open source framework.

Intends to stay very close to the feeling of playing Vic2 but still undertakes slight changes.

Also provides the Historical Project Mod for itself here.

Overall looks really good and quite far along in development!

Open source reimplementation of the SAGE engine (C&C Generals, Zero Hour, LOTR BFME, etc).

Early early stages, not playable yet and only supporting windows for now unfortunately. But could be good to keep an eye on over the long term.

Another opensage implementation is here which is even less far along and has fewer eyes on it, but compiles on Windows, Mac and Linux.

Command the forces of The Project in a battle to rebuild the world after mankind has been nearly destroyed by nuclear missiles. A 100% free and open source real-time strategy game for Windows, macOS, Linux, BSD+ - Warzone2100/warzone2100

Open source C&C Red Alert implementation!

A Victoria2-alike 'map game' and RTS (grand) strategy. Seems relatively unfinished in its current state?

Have not dug deep into it at all yet.