Mindustry is neat if you like tower defenses and factory/automation games.
[edit] Though this isn't exactly a game, I thought I'd mention it: The Powder Toy. Really fun physics-ish sandbox game if you like just playing around with little particles or making electronics or nuclear powered fireworks shows, etc.
Nethack! It's basically the original roguelike, if you liked shattered pixel dungeons then you'll like nethack. There are also several other open source rogelikes to check out as well... including the original Rogue!
Mindustry and Shattered Pixel Dungeon, I played those a lot of my phone but haven't done so in quite a while! I think both of those are available on desktop Linux too, Shattered Pixel Dungeon even works on my Linux phone.
It's pretty basic, but I'm going to say SuperTuxKart as a must if you are into kart racers (especially since I don't know any others).
I also recommend the still in development SuperTux Advance by Kelvin Shadewing. It's similar to just plain old SuperTux but has unique levels and more playable characters that will have different abilities (probably still mostly locked behind debug for them).
Beyond All Reasons. It’s a spiritual successor of Total Annihilation and Supreme Commander using the Spring engine.
It’s also a great game in its own right, especially the various advanced unit positioning and selection features had me amazed. The graphics are good and the performance with many units is finally very good too, after some patches last year (or two years? I don’t remember).
I am on Mobile (aka lazy), so no link. Simutrans for me, beats every other transport sim. The organic growth of cities and industries that happens while you build and optimize your transport systems is one of the many features I like so much.
Tremulous is a free and open source asymmetric team-based first-person shooter with real-time strategy elements. The game features two opposing teams: humans and aliens. Each team must attack the enemy’s base and team members while defending their own base.
https://tremulous.net/
When I did play foss games, I played battle for wesnoth, teeworlds, minetest, super tux csrt and openarena. Lsst one might be dead due to being a mainly multiplayer game.
@sleepybisexual You know what, I could link my articles from a few years ago. It's been a while, so some of newer Open Source games are missing from these lists.
Other than the Valve made stuff, I think my favorite Source engine game is Dark Messiah of Might and Magic. Kicking orcs off cliffs or into spikes is super fun and the basic movement on the engine is pretty much the best FPS has.