What are your favourite open source games?

https://www.openttd.org/
Open source game based on Transportation Tycoon Deluxe.
- I wish it had a modern GUI though. The old one is almost unusable even though they've made an attempt to implement DPI scaling.

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.
- Github link for convenience
  https://github.com/Anuken/Mindustry
- This was the first thing I thought of when I saw the title of the post. Mindustry is awesome, I got so addicted to it for a bit haha
- Mindustry is great! I've put a ton of hours into it and I've gotten my kids into it. Super active dev and good community.
  I prefer it to factorio

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.
- I'm surprised hardly anyone else has mentioned Wesnoth yet. I last played it over a decade ago, so I'm not sure if it's still as well-known now as it used to be, but it's one of the highest-quality open-source games out there.
  It's a turn-based fantasy strategy game that feels like a combination of Advance Wars, Fire Emblem, and D&D. And I suppose it was a lot more unique back in the 2000's when there weren't a dozen indie games out there that fit the same description.

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!
- Are there good UIs/tilesets for Nethack these days?
  
  I enjoyed one on my phone called "Pathos" but I don't know how open it is.

Super Mario Bros. X (TheXTech)
Sonic Robo Blast 2
SuperTux
SuperTuxKart
3 out of 4 begin with "Super"!
- Ooh, those all look so fun! Thanks for the links! :)

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.
- Yeah, Mindustry is cross platform and available on Linux too. Love mindustry, such an awesome game!
  
  It took me two or three attempts to really try and grasp the scale (one of the most complex games I ever played on my phone) but once I did I played it for countless hours! :)

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

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 need to get into this -- I was a massive fan of TA
  
  It's honestly so good, it's peak on army control, huge online community, and active balance/development. Can't recommend it enough.

Unciv is open source civ5

OpenMW
- Doesn't that need the original, very much not open source, data files?
  
  yes, i think they're working on support for custom games though

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.
- Omg I looked at veloren and I'm going to have to try that... I spent so many hours in cube world when it first went early access (before the dev went radio silent for 5 years and then released on steam, lol)

veloren! also my favorite game too to play with my friend
- Currently downloading and making an account
- Is this an inside joke, a technical bug or a mistake? Your link directs me to this post, going in circles.
  
  looks like a mistake

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

ScummVM works really well in linux, if youre a fan of classic point and click adventure games.
- Running on that VM, I really enjoyed Flight of the Amazon Queen a few years back. Warning: it's an oldie containing lots of sexist moments - I'm not sure I would enjoy it as much today.

Xonotic is one that i would reccomend heavily, and it's an arena shooter
- I remember trying it a long time ago, and it was pretty fun.
- Xonotic was awesome. Warsow (Warfork now) was super fun back in the day. There were legit tournaments that went on with twitch sportscaster calling the play by play on 1v1 tourneys. Not sure how big the player scene is these days.

SuperTuxKart is really good with friends.

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.

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

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

No one is saying Shattered Pixel Dungeon?
Because I say Shattered Pixel Dungeon.
- I played vanilla pixel dungeon years ago and recently picked it back up. What's the difference with Shattered?
  
  Shattered has added a lot more polish, some story elements, some more interesting bosses and challenges, and removed the frustrating weapon breakdown element. It's under active development, whereas I think the vanilla game is now a dead fork.
  
  While vanilla was promising, it always had that "school project" kind of vibe. Shattered, intead, is a full fledged, really polished game with active development.
  Among the new features there are new sub-missions (with a new one in development right now), new enemies and bosses, a new playable character and re-balanced mechanics overall.
  Stongly suggested, you can even find it on f-droid

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.
Edit to add: Forgot to mention it is still receiving updates!

I'm a big fan of Thrive!
- I haven't tried thrive in a while. I hear they've switched engines again, to Godot this time. Is it playable? Is it fun? Do they have more than the single-cell phase?
  
  I've spent a solid 30-40 hours playing it since December last year so I'd say it's pretty fun and playable! IIRC it has prototypes up to the interplanetary phase though only the single-cell and multicellular stages were really playable when I last gave them a go.

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/
- Currently downloading but is it still active? Is there still a player base?
  
  I'm not sure. I used to play more but my free time has fallen off.
- Reminds me of Natural Selection 1/2. Loved those games and I wonder if this was their inspiration. And of course they ended up making Subnautica with their expertise in creature design.

If Roblox counts as a game instead of a platform, then I guess Minetest also counts as an open-source game.
- Roblox isn't a game. Minetest is an engine (providing it's own APIs that game developers use to develop games), and a platform to distribute them. In a sense, Minetest is like Unity or Godot. Games built on Minetest include MineClone2/5, Nodecore, Exile etc.
  
  Yeah, but truth is a lot of people do consider it a game.
  
  Yeah, let me open up Unity for that vanilla gameplay experience, oh wait...
  Also, are wc3 and hl also not games because they were used to develop and distribute games that ended up being more popular?

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.

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.

@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
- There is a complete set of free assets, in the form of Freedoom. Also, the official Steam version is closed-source, so make sure you're getting a free source port. (Most distros have a few in their repos, and Freedoom+GZDoom is also in Flathub.)
  
  @ipacialsection Ah yes, I should have mentioned Freedoom! Thanks for the link, totally forgot about it!

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.

@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.
https://thingsiplay.game.blog/2020/03/13/open-source-drm-free-games/
https://thingsiplay.game.blog/2020/05/19/open-source-and-alike-games/
- I forgot half these games existed. Thanks for sharing!
- Jeez, thanks for this blast from the past. I remember a lot of these from my early Linux experiments over a decade ago.

- Another shout-out for Sonic Robo Blast! I absolutely love that game.

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/
- There are many things that use a license that isn't GPL that area still open source. I haven't looked into this one past the fact that it's a custom license though. So it'll probably depend on your definition of open source.

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

https://github.com/divVerent/aaaaxy

FreeOrion

I can recommend colobot if you like to automate things with code.

Cataclysm DDA is amazing if you are into this stuff.

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

Definitely Neverball. My kids and I spent so many hours in it.
OpenTTD is good, so is TuxKart, but both have better closed-source alternatives. I don't think Neverball does.

OpenTyrian (likely available in your package manager)

I really like MineClone2 (a Minetest game).

Crack Attack! is a loving parody of addictive tetrislike games that is also an extremely addictive tetrislike game.

Big fan of Quaver, its a community-driven, and open-source competitive rhythm game.
https://github.com/Quaver/Quaver

I liked oolite (space trading and combat game, also DRL (aka doomRL) and cataclysm DDA , also there is The Ur-Quan Masters megamod (A 2d space game)

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.
- Think you missed a word in OP's post. He's looking for open source games, not open source games
  
  Yeah... I deleted my post as soon as I realized my mistake, but I have an issue with them sometimes not actually being deleted, keeping my dumb mistakes there forever 😩
- Open source != Source engine

Lix – https://www.lixgame.com/
FOSS Lemmings, with many great features (like multiplayer) and active development.