Battle for Wesnoth is a personal favorite of mine that I haven’t seen mentioned elsewhere in the thread. It’s a pixel art, hex-grid turn based strategy game with fantasy armies. It’s mixed with mild RPG elements, and there’s tons and tons of campaigns and fanmade content accessible directly through a mod browser in game.
The other games I enjoy that I see being mentioned here are in the same vein. BAR, Warzone 2100, OpenTTD. And the open mods based on the STALKER engine, like Anomaly. I love those.

Beyond All Reason

https://www.beyondallreason.info/

It a grand scale RTS game with robots and insects loosely inspired by Total Annihilation.

Dungeon Crawl Stone Soup Love this traditional roguelike!

Dungeon Crawl Stone Soup for me. A classic roguelike with active community and regular updates as well as several forks. It also has a good amount of playable races. From classics like elfs or orcs and dwarves (RIP Mountain Dwarves you were too good for this game) to funky races like Vine Stalkers (sentient parasitic plants) and Armataurs (centaurs but with armadillo parts instead the horse ones) to cats and octopuses.

And the gods you can worship in the game are also very diverse. How about a slime god? Or someone who wants you to wear as much cursed equipment as possible in exchange for knowledge? Or maybe the one who wants everything to move veeery slooow and rewards you for killing mobs that are faster than you?

Warzone 2100. It reminds me of the older Command & Conquer games with a ton of upgrades and customization for your units.

OpenTTD is my drug. I’m addicted

Cataclysm DDA Is THE open world zombie survival experience. So long as you like obtuse roguelikes with nearly no graphics.

Although Im obviously biased what with being a dev and all.

Mindustry is an awesome strategy game that combines Factorio with tower defense.

Sonic Robo Blast 2 Kart (ridiculous name I know) is one of the most underrated games ever. It’s a super polished, high quality Kart game with an insane amount of mods and maps. The online isn’t that populated but the singleplayer time trial is TONS of fun. If you like hunting for medals, it can be a pretty deep rabbit hole. There’s a sequel in development but no release date.

GzDoom is the most popular Doom source port and there’s a practically infinite amount of user generated content. If you don’t know where to start I highly recommend Doom: The Golden Souls Remastered and Castlevania: Simon’s Destiny. They’re good enough to be premium games.

Cataclysm: Dark Days Ahead

Did you guys tried Endless sky? Awesome game if you love space

Mindustry is a really fun automation/tower defense game

Pokemon Emerald 😉

Teeworlds. There are some great community made modes for it like FNG and DDrace. Be sure to download it from ddnet.tw to access to extra features created by the community.

Beyond All Reason is AMAZING. I can’t say enough about it. Its my favourite game, and totally annihilates (!) the competition.

Though I guess the competition is pretty small for large scale RTS. Maybe:

• Total Annihilation
• Supreme Commander
• Ashes of the Singularity
• Sins of the Solar Empire (sorta)

BAR has fixed all the UI/UX problem, scales well, has a big community, is free, fun, and super deep and complex.

Cataclysm Dark Days Ahead has been mentioned so I’ll add OpenMW to the thread. It’s a open source engine rewrite of Morrowind and requires the base game to run Morrowind but it has the potential to be used to create new games and the devs are looking to expand it to allow the creation of ARPGs as well.

Another worthy mention is Endless Sky, it’s an open source game inspired by the classic space trading and combat game Escape Velocity.