You might want to try Veloren and Battle for Vesnoth.

Unciv - this one's especially good on mobile

FreeDoom

Sonic Robo Blast 2 if fangames are acceptable

Edit: Since I mentioned FreeDoom -- Gzdoom acts as a sort of platform for Libre FPSes. Ashes 2063 and Wolfenstein Blade of Agony for instance.

To name a few: AssaultCube, Battle for Wesnoth, Cube2: Sauerbraten, FligthGear, Freeciv, Freeciv21, Nexuiz Classic, OpenArena, OpenHV, OpenRA, OpenTTD, Remnants of the Precursors, SpeeDreams, Stone Kingdom, SuperTux, SuperTuxKart, Unciv, Urban Terror, Veloren, Warozone 2100, Widelands, Xonotic

P.S. It may be that not all of them are FOSS, but they run natively on linux.

I've played and enjoyed:
OpenTTD
OpenRCT2
OpenClonk
Hedgewars
Foobillard++

I've also been looking at Tabletop Club but haven't played with it much yet.

BAR - Beyond All Reason, for Total Annihilation/Supreme Commander style RTS, learned about this one in another thread here on lemmy

Zero-K - similar premise, plays different from BAR, also the graphics seem less demanding

OpenSoldat - 2D arena shooter. For anyone that never played or saw something similar, think of multiplayer maps of halo, quake or unreal, but if it was a 2D platformer

Not quite open source, but Daggerfall Unity is a FOSS update to the engine for Daggerfall, a game which Bethesda has made free for years now. You can get the game off GOG, too.

There is !opensourcegames@lemmy.ml here on Lemmy.

Cataclysm: Dark Days Ahead

https://cataclysmdda.org/

Ones I've played (mostly when I was younger) and enjoyed a lot:

Teeworlds

Warmux

SuperTux

Cube 2: Sauerbraten

Games which I have played for more than 100 hours.

Shattered Pixel Dungeon: A traditional rougelike dungeon crawler.

Rhythia or Sound Space Plus: Rythm-based aim game.

Taisei: A Touhou FOSS Fangame.

Osu!: A rythm based game.

There is a Wikipedia article about this: https://en.wikipedia.org/wiki/List_of_open-source_video_games

It is, however, vastly incomplete, as entries without "reliable sources" get deleted. Mind that linking the source code repository, the steam page, the license file and news about a game going open source are not enough to count as "reliable source".

Wikipedia has a list, it may not have every single one but it should have most of them.

https://www.minetest.net/ and https://libregamewiki.org/VoxelLibre

Endless Sky is an amazing one. I've put tons of hours into it. It's a top down 2d space trading/fighting game, very similar to Escape Velocity if you've ever played that game.

Open transport Tycoon deluxe. Been going for years and it's still great.

I'm a fan of Freedroid Classic, a FOSS remake of a commodore 64 game called "Paradroid". You're a robot on a deserted space ship full of other malfunctioning robots, and you have to hack / shoot all of them. You start out as the worst robot, but if you encounter a better one, you can hack it to take over its body -- if it doesn't kill you first.

Takes a few runs to get the hang of it, but it's a lot of fun.

Adding to what others have said If you are looking for or interested in game engine clones https://osgameclones.com/

Nikki and the Robots, it's written in Haskell

Supertuxkart and supertux

also check out libregamewiki its got lots of free games

Here is also a website where you can see if there is an open source port to non-open source games or an alternative that e.g. tries something of its own / based on the gameplay. As an example, the following "clones" are listed for Minecraft: Minetest, Mineclone2 (now VoxeLibre), ManicDigger and more.

Edit: fixed some grammatic fails.

Would reccomend osu!. A cool rhythm game with support for different gamemodes. The lazer version is open source and is on development to ensure it can match all expectations.

ssh nethack@alt.org

Dr. Robotnik's Ring Racers

Didn't see any mention of dungeon crawl stone soup so I'm adding it here

Selaco is a quite recent FOSS game, pased on DOOM.

Every couple of years I throw 40-80 hours into (eg) https://crawl.akrasiac.org:8443/#lobby

its pretty complex and can be hard to learn, but flightgear is super fun once you get the hang of piloting.

Here are some less mentioned FLOSS games, that are excellent quality:

Fillets-ng (2D fish sokoban)

The Dark Mod (3D stealth game, bow and sword)

Crrcsim (3D model glider flightsim, slope soaring)

Rexuiz

https://rexuiz.itch.io/rexuiz-fps#download

I can also recommend Rota, it's a relatively new puzzle platformer. https://snapcraft.io/rota

Does Katawa Shoujo count? It’s not quite FOSS, but it’s a renpy game under CC BY-NC-ND.

BZFlag has existed for a while now.

I haven't played it yet, but a game called Veloren looks really good, and I've heard great things about it.

I spent several years on OpenBSD playing mostly foss games (no WINE support) before I got my steam deck, so I feel fairly qualified with recommending Cataclysm DDA, Xmoto, and Dungeon Crawl Stone Soup. Say goodbye to the next several months of your life. Other cool FOSS games that grabbed me, but not to the same level: Ur Quan Masters (Star Control 2), and endless-sky.