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.

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

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

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

https://libregamewiki.org/

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

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

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.

Xonotic

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.