Widelands, FOSS RTS
Widelands.org
Widelands is a free, open source real-time strategy game with singleplayer campaigns and a multiplayer mode. The game was inspired by Settlers II™ (© Bluebyte) but has significantly more variety and depth to it.
Unfortunately the graphics in this game are pretty awful. The textures are quite flat and rely on a sad looking dark palette.
There’s also the Settlers 2.5 which is actually using the original game‘s graphics and looks way nicer: https://www.siedler25.org/
Need to check this out.
I am one of those people that doesn't mind even outright bad graphics as long as the game is fun. :)