Gaming

Mindustry is a really fun automation/tower defense game

OpenTTD is good

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

Beyond All Reason is AMAZING. I can't say enough about it. Its my favourite game, and totally annihilates (!) the competition.

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 !

Beyond All Reason

https://www.beyondallreason.info/

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

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!

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

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.

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.

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.

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

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/

Dungeon Crawl Stone Soup Love this traditional roguelike!

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

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

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

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

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

Teeworlds. There are some great community made modes for it like FNG and DDrace. Be sure to download it from ddnet.tw to access to extra features created by the community.

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.

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.

Not quite what you are asking for, but here's a list of source ports of commercial games, most of which have Linux ports.

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

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 :)

Unciv civilization 5 replica

Edit: added description

Tux Racer. Hands down.

Cataclysm: Dark Days Ahead

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.

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.

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 :)

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.

Pixel Dungeon! Great rogue like mobile game

OpenTTD is my drug. I'm addicted

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/

There's not a whole lot of activity around it these days, but Zelda Classic is an engine for Zelda 1 with a number of additions and improvements, that allows for custom maps, tilesets, scripts, items, enemies, etc. etc.

I've whiled away an unreasonable amount of time on quests that are on par mechanically and quality-wise with Link's Awakening and the Oracle games. Shame I can't get the tarball to work properly on Steam Deck, it's a great fit for that hardware.

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)

If you didn't already suggest it, I would have suggested Veloren!

OpenRCT2, eDuke32, the various Doom and Quake games

Also ScummVM is open source and supports many games

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

Veloren

osu!lazer