Here are some popular choices:

• Xonotic, an arena FPS in the style of Quake 3 and Unreal Tournament that has a thriving online scene – including non-standard modes like Defrag
• Hedgewars, an artillery game that’s basically “Worms: Armageddon”, but with some annoyances removed (like stray solid pixels get automatically removed, jumping is easier to do, etc.)
• Teeworlds, a fast-paced 2D multiplayer action game
• The Battle for Wesnoth, a turn-based strategy that (unusually for a PC title) adopts a gameplay structure more reminiscent of console TBSs (comparable to Advance Wars or Fire Emblem, but not a clone of either)
• Minetest, an attempt to make a Minecraft clone that has since turned into a very flexible engine for block-based games

And personal suggestions:

• Mindustry and shapez.io have already been mentioned by another person here, they’re worth checking out
• Shattered Pixel Dungeon is a newbie-friendly roguelike
• speaking of roguelikes, NetHack is a classic, although not as user-friendly and way more difficult
• Simon Tatham’s Puzzles, a collection of small games that all implement classic puzzles (Solo is sudoku, Pattern is “Japanese crosswords” aka Picross, etc.)
• LAN Master (video, download), a legit public-domain open-source homebrew NES game where your goal is to connect all computers into a network by rotating pieces – obviously you’ll need an NES emulator for that, FCEUX, Mesen, Mednafen, ares and Retroarch are good FOSS choices for that

https://libregamewiki.org/

I haven’t seen anyone mention Endless Sky.

It’s in ongoing development, but there are many hours of content. You fly your ship around, you can be a trader selling goods between planets, be a pirate attacking other ships, or fight pirates and sell their stuff. There are lots of other quests, including a major and lengthy story line quest series and other minor side quests with multiple linked parts.

There’s also an unofficial port to Android available on F-Droid here.

I’ll be the nerd and just drop a wikipedia article :p

https://en.wikipedia.org/wiki/List_of_open-source_video_games

The devs of the rhythm game osu! are creating a foss remake of the original, which is known as lazer.

There’s an open source remake of Revolt, which was a 90s racing game using toy cars and power ups. https://rvgl.org/

Mindustry and Shapez.io. If you are a fan of Factorio, I highly recommend these two games.

Mindustry is a combination of resource management automation/tower defence/light RTS. https://mindustrygame.github.io/

Shapez.io is pure management automation but with a quite aesthetic vibe.
https://shapez.io/
https://github.com/tobspr-games/shapez.io

Both are also available on Steam if you want to support the developer.

Lately I’ve been playing some AAAAXY, it’s a cute/confusing puzzle-platformer.
And I’d be a fool if I didn’t shill my favourite game, Hase. It’s a “gravity based artillery shooter.” It’s basically Worms/Hedgewars but in space with prettier graphics and fun multiplayer!

One Hour One Life seems to be a relatively popular one.

OpenTTD

OpenTTD is an open source simulation game based upon Transport Tycoon Deluxe

You build a transportation network using buses, trains, planes and ships to connect cities and industries. Contains a huge variety of mods called “NewGRFs”, which can do anything from adding real life trains to completely overhauling the supply chain.

deleted by creator

Veloren

Veloren is an open world, open-source multiplayer voxel RPG inspired by Cube World, The Legend of Zelda: Breath of the Wild, Dwarf Fortress, and Minecraft, written in Rust.

@cavemeat@beehaw.org @gaming@beehaw.org Lately I’ve been playing some AAAAXY, it’s a cute/confusing puzzle-platformer.
And I’ve be a fool if I didn’t shill Hase, a “gravity based artillery shooter.” It’s basically Worms/Hedgewars but in space and with prettier graphics and really fun multiplayer :gutkato_kosmovojaĝisto:

Xonotic

SuperTuxKart is always a classic, and GLTron is great too, it’s a FOSS Tron clone with multiplayer.