Gaming

Mindustry is a really fun automation/tower defense game

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

OpenTTD is good

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 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.

Shattered Pixel Dungeon ! And its origin, Pixel Dungeon.

A really nice rogue-like, among the best Android games I know, it’s still fun after all those years and very actively developed !

Veloren is inspired by games such as Cube World, Legend of Zelda: Breath of the Wild, Dwarf Fortress and Minecraft.

You can play single player or multi-player, standalone or use an online server, or even host your own server in a Docker container, or on a Raspberry Pi. Plenty of options!

You start by creating your character, you can collect items for your inventory, you can craft items, there are weapons and combat, you develop skills, can tame creatures, you can trade with merchants, you can socialise, and lost more. There is no single, specific goal or focus, and the idea is to keep exploring and have adventures.

The game is community driven and actually updates quite regularly.

It is clearly no clone of Minecraft. It is fun and adventure!

See https://veloren.net/

I will say that 0 A.D. and veloren are my favourites!

Beyond All Reason

https://www.beyondallreason.info/

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

Have not seen anyone mention one of the best rogue like games out there, Tales of Maj'Eyal

https://te4.org

Base game is free to download and play from their website. Massive world, tons of loot, an incredible number of classes and abilities to combine, it's such a fantastic game that also has native Linux support.

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.

Battle of Wesnoth. Haven't played it in a while, but I remember spending hours playing that at one point.

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.

https://github.com/open-duelyst/duelyst

Duelyst was a very popular online card game that shutdown around 3 years ago. The developers have recently released it into the public domain (via CC0 license).

You can completely selfhost a private instance (repo has a good straight forward guide)

There is a public instance available to try https://duelyst.org/. Though it might be pretty barren.

There is also a closed fork which should have a more active userbase https://duelyst2.com/, the game rules are a little different (draw 2 vs draw1), but should have more users to get a feeling for the game.

The game in a nutshell is basically Hearthstone on a chessboard. It’s very fun and approachable and in my opinion one of the best cardgames right next to MagicTCG :). Hopefully the community for it will grow more and we can have a sort of second renaissance for it :)

Warzone 2100 is an RTS from 1999 that went open source at some point and can be played on just about anything. It's unit design functionality is really cool and I'm a sucker for post-apocalyptic worlds.

OpenRCT if you're counting that style of open source game

CataclysmDDA is an amazing open source roguelike. Has a bunch of tilesets if you don't like ASCII

If you like RTS games similar to Total Annihilation, you should definitely check out Beyond All Reason, it's gorgeous and fun: https://www.beyondallreason.info/

Cube 2 Sauerbraten is pretty good! Free arena shooter with built-in level editor

Unciv civilization 5 replica

Edit: added description

Here's a good start.

Fillets-ng (sokoban puzzler)

Simon Tatham’s Portable Puzzle Collection (sgt-puzzles)

Sauerbraten (FPS)

0 A.D. (RTS)

Holotz Castle (platformer)

Xmoto (motorbike trials riding)

Kobodeluxe (shmup)

Neverputt (minigolf)

Oh I am saving this post! It's a goldmine

I haven't seen any incremental games get posted. They aren't exactly traditional games, but so many of them are open source like Antimatter Dimensions or Synergism. Bitburner is a unique one as an incremental hacking sim, since it relies on actual scripting to grow and automate tasks.

Supertuxkart

Dungeon Crawl Stone Soup Love this traditional roguelike!

I really enjoy Thrive, it's basically a modern and more scientifically accurate Spore!

Oof, I have a huge boner for roguelikes, and most of them are open source. My biggest time sink is still Dungeon Crawl Stone Soup, playable online at https://crawl.develz.org/

Several, but my number one fav so far is Battle for Wesnoth. An old school hex tile turn based strategy game. Like something out of the 90's or very early 2000's.

Great art, awesome music, solid gameplay, and runs on a potato.

My best friend and I play it whenever we want a casual chill, play n' chat gaming session.

It runs great on the Steam Deck too with a few interface tweaks.

Classic roguelikes have the most longetivity for me. Crawl, Brogue, Nethack.

OpenRA has open source slightly modernized versions of Red Alert, Tiberian Dawn and Dune 2000. There's also Tiberian Sun-like and Red Alert 2 mods for it :)

Tux Racer. Hands down.

Dungeon Crawl Stone Soup

Doom :)

Xonotic is a fun quake-like shooter. Highly recommend.

Unciv! Remake of civV for desktop and Android.

Shapez is a fairly relaxing factory game. Pretty minimalistic style but easy to lose tons of time to if you aren't careful.

Endgame: Singularity is a fun (and sometimes very frustrating) small strategy game worth checking out.

The Dark Mod is basically Thief but really well done with a solid amount of (fan) missions to choose from. Speaking of I should play some of it again.

Pixel Dungeon! Great rogue like mobile game

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.

If reverse-engineered Nintendo games count, then Super Mario 64 and Pokemon Emerald whose engine is technically open source.

EDIT: Minecraft Beta 1.7.3 has been reverse engineered multiple times too.

Cataclysm: Dark Days Ahead

Veloren

0 A.D. is the closest experience to Empire Earth I've had on the FOSS side. Great fun!

Some great games already on here, but wanted to share some love for VVVVVV which released its source a few years ago.

I like supertuxkart and minetest