Battle for Wesnoth

This one never gets the love it deserves. It's a fantastic turn based strategy game with multiple campaigns and storylines, multiplayer, and campaign design tools. It's an old project (started in 2003, IIRC), but it's still fantastic

• http://www.wesnoth.org/
• Steam: https://store.steampowered.com/app/599390/Battle_for_Wesnoth/

Hedgewars

Basically, this is Worms but with adorable little hedgehogs instead of, well, worms. Single player is okay, and it has online multiplayer I guess, but the real fun (just like in Worms) is local multiplayer. Also, it has Portal Guns. There's really no downside to this one.

• https://www.hedgewars.org/
• Steam: https://store.steampowered.com/app/2223810/Hedgewars/
• Itch: https://hedgewars.itch.io/hedgewars

Re-volt io and RVGL

This one's a little iffy. Re-volt was a fantastic R / C racing game with bright graphics, fun tracks, excellent controls, and a killer soundtrack. For good or ill, it was put out by Acclaim, which self destructed in 2004. The Re-volt fan community, however, doesn't know the meaning of the word "quit."

Nowadays, you can join a lively community with regular online tournaments of the game. There's a new cross platform engine called RVGL (that's Re Volt Game Launcher), and metric tonnes of mods and fan content. You still need the original game's assets, though, which is where it's dicey; they're technically abandonware not open source, unless I'm mistaken.

Anyway, links!

• https://re-volt.io/
• https://rvgl.org/

Unciv

This project's aim is to be Civilization V, but with more abstract visuals, and, or course, free. In short, it's FreeCiv, but Civ V instead of II and a UI from the 2020s rather than the 80s. (Not throwing shade here; FreeCiv is an amazing project that is exactly what it wants to be!)

• Github: https://github.com/yairm210/Unciv
• Itch: https://yairm210.itch.io/unciv

If that's not enough to keep ya' happy, I know a few more, but they've mostly been covered by other folks here.

Edit: formatting