Endless Sky.

It's an open world 2D spacefaring game, with a good core story and more being added by the surprisingly-active dev community.

Not without its flaws, but anyway I think it's a fantastic game in its own right. And it's both "finishable" and under continued development & expansion.

My favorites:

• OpenTTD: Old transport tycoon game, but really fun especially in multiplayer
• Minetest: Minecraft-like voxel game engine, which means there a many different game modes
• Beyond All Reason: Great RTS, also great in online multiplayer or with some friends. (Disclaimer: Possible unfree licenses for artwork)
• Dwarf Fortress (free but not open source, thanks for telling me that one): not everyone’s cup of tea, but sometimes you just get stuck in it for hours on end

Battle for Wesnoth

This one never gets the love it deserves. It's a fantastic turn based strategy game with multiple campaigns and storylines, multiplayer, and campaign design tools. It's an old project (started in 2003, IIRC), but it's still fantastic

• http://www.wesnoth.org/
• Steam: https://store.steampowered.com/app/599390/Battle_for_Wesnoth/

Hedgewars

Basically, this is Worms but with adorable little hedgehogs instead of, well, worms. Single player is okay, and it has online multiplayer I guess, but the real fun (just like in Worms) is local multiplayer. Also, it has Portal Guns. There's really no downside to this one.

• https://www.hedgewars.org/
• Steam: https://store.steampowered.com/app/2223810/Hedgewars/
• Itch: https://hedgewars.itch.io/hedgewars

Re-volt io and RVGL

This one's a little iffy. Re-volt was a fantastic R / C racing game with bright graphics, fun tracks, excellent controls, and a killer soundtrack. For good or ill, it was put out by Acclaim, which self destructed in 2004. The Re-volt fan community, however, doesn't know the meaning of the word "quit."

Nowadays, you can join a lively community with regular online tournaments of the game. There's a new cross platform engine called RVGL (that's Re Volt Game Launcher), and metric tonnes of mods and fan content. You still need the original game's assets, though, which is where it's dicey; they're technically abandonware not open source, unless I'm mistaken.

Anyway, links!

• https://re-volt.io/
• https://rvgl.org/

Unciv

This project's aim is to be Civilization V, but with more abstract visuals, and, or course, free. In short, it's FreeCiv, but Civ V instead of II and a UI from the 2020s rather than the 80s. (Not throwing shade here; FreeCiv is an amazing project that is exactly what it wants to be!)

• Github: https://github.com/yairm210/Unciv
• Itch: https://yairm210.itch.io/unciv

If that's not enough to keep ya' happy, I know a few more, but they've mostly been covered by other folks here.

Edit: formatting

• 0 AD
• Battle for Wesnoth
• Tales of Maj'Eyal

Does open source implementation of proprietary game counts? OpenMW

OpenTTD. IMO mindustry, for example, is better but it's still worth to try OpenTTD

osu! is an amazing rhythm game. Try osu!lazer, it's the new client: https://osu.ppy.sh/home/download

I also like Pioneers and Endless Sky, both space sims.

This might not count, but both Lichess, a chess website, and OGS, a Go website are open source.

There is also Mindustry, but I haven't played it.

!opensourcegames@lemmy.ml is good to share such games.

Thrive

tl;dr: a better Spore

From the site:

Thrive is a free, open-source game about the evolution of life.

From the lowly tidepools of your home planet, you will rise to cosmic dominance. At every step, powerful creation tools and deep simulation mechanics will aid your species’ development. Revolutionary Games is the open-source team of dedicated volunteers aiming to make Thrive a reality.

Our team seeks to accomplish two major goals: create engaging, compelling gameplay that respects our players’ intelligence, and remain as accurate as possible in our depiction of known scientific theory without compromising the former.

MineClone2 is addictive. All that Minecraft style fun, but free and open source. It's pretty comprehensive and solid for the most part.

Warzone 2100, from the community project website and not Steam. Some douche just took the code and uploaded it to Steam themself and does a poor job maintaining it.

OpenTTD, OpenRCT and OpenRA are all great!

Crossfire https://crossfire.real-time.com/ Battle for Wesnoth https://www.wesnoth.org/ Unciv https://github.com/yairm210/Unciv/ Dungeon Crawl http://crawl.develz.org Feudal Tactics https://github.com/Sesu8642/FeudalTactics

One Hour One Life is open source, it is a 2D hand drawn survival game where you have 1 real life houre to live from a baby to an elder and contribute to the player-made society in your life as best you can.

You have to pay for an account on the official servers, but i recommend you do to support the development.

Not sure if the dev accepts community patches or not, but the game is public domain license.

https://en.wikipedia.org/wiki/One_Hour_One_Life

Minetest and several others that I play regularly have already been mentioned, so I wanted to shout out a game I've been following and playing for years, waiting for their 1.0 release: FreeOrion

https://www.freeorion.org/

It's essentially a clone of the Master of Orion series, which are classics in the 4X world.

This is a pretty cool newish one: https://hypersomnia.xyz/

We will play it during this upcoming event: https://onfoss.org/news/2023/08/27/Next-event-September-23rd.html

My PC always has 3 games installed, and depending on the distro I'm currently on they may have come directly from the default repos:

• Battle for Wesnoth
• Warzone 2100
• OpenRCT2

I have lost count of how hours I have in all 3.

Widelands.

Xonotic, if you like very fast paced shooters.

Super Mario Bros. X (TheXTech)

OpenRCT (not totally free but a damn impressive project nonetheless)

People like giving recommendations like Super Tux Kart that haven’t aged well and don’t play well.

What's the issue with SuperTuxKart? I thought that it was great fun when I played it.

Teeworlds is a good game. Unfortunately the player base got very small, even though it's on Steam.

I had fun playing Dink Smallwood (I think the Dink Smallwood HD Version on Android). You run around, solve tasks, and work your way through the story.

It is, as you described, not actually great, but great in a sense that you can have much fun with it. I especially liked the humor.

OpenRA, MineTest, Veloren

Space Station 13 is great

Barotrauma is source available (no commercial, public use, restricted for game modding and contributions) Librerama is dumbways to die style minigames

xevil

When I played open source games around 15 years ago, these were all the same games mentioned. There doesn't seem to be a lot of progress

My favorite: cataclysm dda, flare rpg, oolite.

ScummVM supports many many games. Some people might think it's an emulator, but it's actually a collection of reverse engineered game engines.

eDuke32 and other Build Engine source ports

SuperTuxKart Xonotic Openarena

I'm not giving any of these a chance.