OpenTTD is fantastic. The graphics might give new players pause, but if you like building networks or logistical puzzles, or just like trains, it’s still one of the best in the genre.
I asked this a while ago which is how I discovered Beyond All Reason which has been my FOSS game of choice as of late.
I’d also recommend Naev and Endless Sky (Both are based on the Escape Velocity Series, Naev is getting a 3D PBR renderer in the next release). Mindustry is good fun, actually purchased this one on steam to support the amazing developer. Extreme Tux Racer is a bit of fun and Super Tux Kart seems to get better with every update (did I mention it can run on the Nintendo Switch via homebrew!)
Edit: I forgot about 0ad and Minetest which I used to play a bit of a while back
BAR is great – Total Annihilation was always one of my favorite games from childhood and BAR feels the most like it compared to other spiritual successors like SupCom/FA and other community projects. I actually tried to contribute a couple commits to the project but I don’t think they took them.
Too many amazing moments and memories to even count, always more to learn. I love how the open source nature of the game means there’s many different servers branching off from each other, running their own custom versions of the game. Smaller servers “downstream” pick and choose which features they want to keep when the upstream servers implement something new.
It’s not open source, but I do want to mention Barotrauma here – it’s not totally unheard of, but I don’t think many people realize that it’s a spiritual successor to SS13. Supports a lot less players, but still up to 12 or something on the bigger ships, and it manages to turn the absolute insanity of SS13 into a compelling survival game that still has plenty of goofiness.
I didn’t see anyone mention Warzone2100 yet. An excellent RTS, with a neat research system and unit customisation, and fun campaign. They’ve recently added a couple of new campaigns I haven’t played yet, and have enough ongoing dev work on skirmish/multiplayer that some AIs are listed as “X% win rate in AI matches”.
From a quick video I saw, they seem similar indeed, but I haven’t played Thrive enough. I have only made it to a single-cell prokaryotic organism, lol (which is basically very close to the beginning of the game).
Super Tux Kart (I play it on Android) is NOT one of them. The physics system is bad at some moments, the items aren’t fun to use and some of them ruin the game. The overall game feels amateurish (in a bad way), but one thing that I like is the Windows Car and the drifting. Those are awesome.
I still haven’t tried it, but I’ve heard how great Veloren apparently is. It’s an MMO voxel game that takes inspiration from Zelda: Breath of the Wild and is written in Rust.