Asklemmy

Battle for Wesnoth - a turn based tactics game in a fantasy setting. It's also available on Steam and itch.io. Coincidentally, version 1.18.0 was released yesterday.

Roguelikes: DCSS, Shattered Pixel Dungeon, Nethack

Right now I'm basically playing Beyond All Reason almost every evening. It's a game in the Total Annihilation "tree" of games. A massive scale RTS. I previously played Supreme Commander and Planetary Annihilation, both of which are also inspired by Total Annihilation, but I have to say that BAR is really better than both of them. I almost can't believe it's an open source game. It's still in alpha, but it's been way more stable than most AAA games I've been playing recently.

I've gotten all my friends hooked on OpenTTD multiple separate times

Endless sky. You start off with a single spaceship and fly around the galaxy trading, fighting pirates, and buying/capturing more ships. The devs are still regularly adding new content too.

Unciv is a Civ 5 remake with simplified graphics. Its a lot of fun but the AI is brutal.

Does DOOM count? It's been officially open sourced recently, even though it was already treated as such for decades.Either way, I can't stop playing it, there's 30 years of user created content to go through.

Mine is Mindustry too

OpenTTD is also nice

In my AP Computer Science class in high school, every now and then our teacher would give us a free day. We would sit around playing Unreal Tournament on the LAN.

At one point the teacher decided he wasn’t comfortable with a game with guns in class.

So…I got everyone to install Cube, the open source first person shooter. Specifically a copy of it where I had removed all of the gun models. So we just ran around punching and bullets would fly from our fists. Or, in the case of the rocket launcher, an exploding sofa would fly from our fists.

I don’t know if I would call it my favorite, but we had a lot of fun with it.

Battle for Wesnoth. There's several FOSS games that are pretty good, but BfW I've played the most.

openttd is great

Someone on lemmy a few months ago mentioned this rts beyond all reason. Since then I've easily put 100 hours in. The game is so dam fun.

Another great one is Osu

CDDA probably. One of the more comprehensive zombie survival experiences

Apart some of the already mentioned (mindustry, 0ad, beyond all reasons, zero-k), also minetest (minecraft), destination sol.

l've tried hege wars (worms clone) but found it meh. Sometimes supertux.

Nethack. I remember playing it at university 30 years ago when it was text only. Amazingly, there is a version available for android and its fucking addictive.

Xonotic

Nethack.

0AD, so good

Super Tux and Super Tux Kart

I play DDay Normandy, a quake 2 mod

OpenRA d2k

Minetest

Veloren

Fun games

Not enough shout outs for Super Tux Kart soccer mode. Getting a group of friends playing that is awesome

One of my favorites recently has been playing MineClone2 with some extra mods (can't remember which) through Minetest.

I've been boycotting minecraft since the news of chat reporting potentially coming to Java and MineClone2 is such a good replacement. Biggest gripe I have is not being able to get any of the mods that add sprint to work. Pretty much the only feature not available in MineClone2 (as far as I'm aware) from minecraft that I actively miss.

More recently, I'm a fan of shapez (shapez.io) for my recommended daily allowance of factory building gaming.

Going back further, I have a soft spot for Ri-li, a nice little game about toy trains with very charming music! It was this which set me on an adventure with the mod tracking scene. A surprising amount of open source games use them, it feels like open source music!

Reminded me also of Alex the Allegator 4, a very original 2D platformer

Doom, or, gzdoom! You can use it to play the original Doom 1 and 2 games with modern mouse and keyboard controls on HD displays. And there's an endless number of fan made level packs to try.

As for open source AND free / open resources, there's Freedoom 1 and 2. I occasionally play them, they're fun, but a little lacking in stage design.

Open RCT 2

mindustry. I've kinda quit playing games. But still play mindustry on my ipad. :)

Morrowind with vanilla OpenMW

I like Ltris, Gweled, and Frozen-Bubble. I like simple games. I wish there was a good pacman for Linux too.

minetest is pretty cool. Highly decentralized which is nice. Very flexible, it's just a game engine essentially.

Sauerbraten

I like Simutrans, which is basically an OpenTTD competitor with more complexity but an uglier interface. Sadly development on it has been fairly slow, at one point there was a one-way road patch but it’s since been abandoned.

Fellow Mindustry fan!

Probably the Ur-Quan Masters.

Chess

Go / Baduk.

However the white stones are severely overpowered, and the devs refuse to do a rebalance since around the last 4000 years!

There are a lot of great responses on here! I've been playing (at least until my steam deck shit the bed two days ago) around with OpenRA and Minetest.

Star Ruler 2, the company closed down and open sourced the game, it's a space 4X real time, has pretty good fleet combat and u can design ships, i think wasn't popular because the resource management and card system is a bit weird and harder to understand than other strategy games, but pretty fun after u get used to it...

Nexuiz, or whatever they call it now after the original dev trademarked and sold the brand.

Edit: Xonotic is the name of the active fork.

Veloren all the fucking way

Angband