Free and Open Source Software

https://www.openttd.org/

Open source game based on Transportation Tycoon Deluxe.

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.

0AD! It's similar to age of empires

https://play0ad.com/

Not a very original answer, but I still enjoy The Battle for Wesnoth after many years.

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).

• Super Mario Bros. X (TheXTech)
• Sonic Robo Blast 2
• SuperTux
• SuperTuxKart

3 out of 4 begin with "Super"!

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).

ScummVM works really well in linux, if youre a fan of classic point and click adventure games.

Currently Endless Sky! Although I haven't gotten far, and playing without mods, I love the vibes

If Roblox counts as a game instead of a platform, then I guess Minetest also counts as an open-source game.

OpenMW

veloren! also my favorite game too to play with my friend

Unciv is open source civ5

@sleepybisexual DOOM! The source code is open source, but the assets aren't. So this might be a bit of cheating reply. https://github.com/id-Software/DOOM

I've dumped more hours into OpenTTD and Zero-K than I care to think about

I haven't played in over a year, but I really spent some hours in Veloren the last time I picked it up!

Xonotic is one that i would reccomend heavily, and it's an arena shooter

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.

I highly recommend playing Doom with the Crispy Doom source port and the Freedoom data files.

For something more modern, Xonotic is about the best-looking FOSS game out there and an excellent multiplayer arena shooter.

Cortex Command Community Edition

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.

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!

Warzone 2100. 3D RTS from the late 90s that was open-sourced after the studio went belly-up. Fantastic game, runs on even ancient hardware.

In terms of games I actually play, Cataclysm: DDA (even if I still suck at it after hundreds of hours).

In terms of technical/visual impressiveness, Veloren.

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/

I'm a big fan of Thrive!

SuperTuxKart is really good with friends.

VVVVVV. It's not GPL though, so technically it's not open source, but source-available.

Note that while the source code is available, the assets is not, so you'll still need to buy the game here: https://store.steampowered.com/app/70300/VVVVVV/

• Doom Roguelike if you want a roguelike with simple, action-oriented gameplay.
• Cataclysm: Dark Days Ahead (lemmy instance) if you want one with complex, simulation-oriented gameplay.

FreeOrion

I really enjoyed Minedustry. Also, does Katawa Shoujo count?

Cataclysm: Dark Days Ahead is pretty astounding, offering an incredible level of detail and degree of freedom, plus just lots of zombies to fight.

No one is saying Shattered Pixel Dungeon?
Because I say Shattered Pixel Dungeon.

https://github.com/divVerent/aaaaxy

I haven't messed with an open source game since early Sauerbraten. I gave up because of the crazy learning curve to git gud.