Gaming

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

Surprised it's not mentioned here, but Bzflag.

Super fun tank shooter game that doesn't take much to run, and reminds me of a cross between the very old bolo game and Mario kart's battle mode.

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

I've had a ton of fun in Red Eclipse over the years.

Aside from the other choices already mentioned (especially Doom stuff, FreeDoom ftw) any of the free games derived from the S.T.A.L.K.E.R. games and early builds, mostly Anomaly and Lost Alpha. There are also a few stabilized versions of the Build 1935 alpha out there which are worth playing and probably a bit more approachable for most people.

OpenRCT2, eDuke32, the various Doom and Quake games

Also ScummVM is open source and supports many games

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.

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.

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/

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.

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

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 :)

0.A.D., Veloren and Xonotic are my favorites. Also Sonic Robo Blast 2 is freaking awesome

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.

The only one Ive played for any length of time is freeciv, which as the name implies is based of the civilization games, mostly civ2. It's a bit old looking these days and setting up multiplayer has always been a bit fiddly, but the gameplay is pretty fun imo.

I am a big fan of Neverball

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

Oh and I'll single out The Ur-Quan Masters as an older source port of Star Control II. Old enough that the port itself is nostalgic for me, although it's still being updated by the looks of it.

Minetest!

https://www.minetest.net/

Does City of Heroes count? It was closed source but was opened...

Puzzle

• Rocks'n'Diamonds - Digging for diamonds while avoiding boulders and enemies
• Enigma | github - Marble puzzles (and much more) inspired by the classic game Oxyd
• GNU Robbo - Solve rooms (puzzles) with your robot

Space

• Oolite | github - an open-world space opera (inspired by classic game Elite)
• Kobo Deluxe - 2D scrolling space shooter, destroy star bases with your ship
• Open Tyrian - Arcade-style vertical scrolling shooter

I still jump into Quake 3 DeFRaG sometimes. It's a movement game, probably one of the earliest examples of a game / mod where the primary focus is on movement as opposed to shooting or some other mechanic. Its mere existence heavily inspired a lot of source engine movement communities like surf, bhop, kz, etc.

Here's a good video showcasing what DeFRaG movement looks like. Each map is ran on two variations of the physics. One is VQ3 (vanilla quake 3) and CPM (Challenge ProMode Arena, which was originally a Quake 3 mod that heavily altered the physics to make it more fun for professional play). While there's no one DeFRaG game, there's a shitload of open source mods to the original game that modify the physics and allow you to join servers running DeFRaG maps. Some of them even hook up to the online database allowing you to register times without recording your runs and submitting them the traditional way. There's usually a new open source defrag mod rolling out every 3 years or so, and each one builds upon the successes of the previous one. The one I currently use is iDFe which I think came out around 2018 or so? I'm pretty sure there's a new one, and there are other games in the works in both modified Source Engine and Unreal Engine 4 that are looking to emulate the VQ3 and CPM physics for modern engines.

The community is pretty dead, but it rises from the ashes every few years for DFWC (the video I linked above was DFWC 2017). You can always play any one of the thousands of maps in singleplayer too.

edit: I should add, it's not totally open source. You still need to own a copy of Quake 3 (or visit the high seas, at least) because each DF mod requires you to verify your ownership of the game by using a valid CD key. There is a "universal" developer CD key, but I don't remember what it was. When you download defrag you're effectively downloading quake 3, but it's bricked if you don't have the CD key.

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

Warzone 2100. It reminds me of the older Command & Conquer games with a ton of upgrades and customization for your units.

Cataclysm: Dark Days Ahead

Veloren

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