Great suggestions in this discussion! Rather than adding my favourites, I will add some resources that list more games.

• Libregamewiki: it is really comprehensive (sometimes too much, including even not-so-good-games). They care about licencing and is is very easy to browse, top-notch for me.
• Open source games: a more relaxed repository, with lots of material.
• bobeff open source list: this is curated, which means that there are not so many games but each and every one is stable, good, maintained.
• Arcane Cache: a fantastic blog with reviews of libre games — or more precisely, underground games, there is a lot of discussion on how gamedevving philosophy too. The reviews are always in-depth and allow you to experience the games on another level, and each game is a small jewel in its category. Strongly recommended!

Some of the most polished open source games are: 0 A. D., Xonotic, Beyond All Reason, Battle for Wesnoth, Mindustry, Minetest, Thrive. These are all great games open source or not. I'm sure I'm missing a few, but these stand out to me for their completeness and polish.

Battle for Wesnoth is the one that comes to mind first. Turn based strategy game.

Dungeon Crawl Stone Soup classical roguelike, as in actually like Rogue/nethack. Can be played with ASCII or graphical tiles, and you can play in a browser window with scoreboards and a spectator mode.

Mindustry is a favorite of mine

• Xonotic is a great old school, Quake style FPS with an active community
• 0 A.D. is a pretty polished historical RTS
• The Battle for Wesnoth is a classic turn based strategy
• OpenRA is an open implementation of the Red Alert engine that's got an active community

Veloren is based on Cube World and it's an RPG with server builds and everything

Unciv is a decent Civilization clone

• osu! (The new lazer client, the old one isn't open source)
• Veloren

OpenTTD is more a strategic game,but it's great. OpenRCT sadly requires the graphics of the original RCT2, so it's not really open source. But also a favorite.

Cataclysm: Dark Days Ahead, has a really strong community behind it. Be warned, it is an intensely deep simulation (as in dwarf fortress), that is often criticized for it's tediousness. But that gameplay element critique aside, it's a fantastic example of a community developped game.

Unciv if you enjoy sacrificing hours of your life :)

0AD is great. And whilst not a game really, FlightGear is the best you'll get for a FOSS flightsim. And really, for the prices that some X-Plane/MSFS planes and other addons go for, it's actually proper good.

I'm surprised that no one mentioned super tux kart yet

Ive been playing a lot of MineClone2 which is a modpack for Minetest!

Amnesia: The Dark Descent and Amnesia: A Machine for Pigs got open sourced if that counts.

Cataclysm: Dark Days Ahead, or if you like something a little less complex, Cataclysm: Bright Nights, are fantastic roguelike games. Proper traditional, turn based, tileset ones.

Imagine a mix of Fallout and The Walking Dead, that's what it's like. It's superb. The possibilities are endless

A lot of good tips here, but you should have a look at Endless Sky! Really cool space game
https://github.com/endless-sky/endless-sky

OpenTTD is a pretty cool transportation and train building game

I liked oolite and doomrl .

0 AD is a solid AOE2 clone

Forge, a Magic: The Gathering rules engine available both for Windows and Android. Various formats, good AI, offline and online play, and now even has a Shandalar-like mode.
#MTG #MagicTheGathering #Forge #Shandalar

Warzone 2100 is an excellent RTS that is open source. I've never played a game with a bigger and more complex skill tree. Has an interesting development history in that it was originally a Playstation game, but was completely open sourced once the company went under. It has been kept alive by fans ever since.

Havent seen endless sky mentioned here, it is an old school space trader sim (think scape velocity or a 2d freelancer)

Neverball is my favorite. It's a Monkey ball clone.

Shattered Pixel Dungeon Roguelike, very addctive :)

For mobile games there is Shattered Pixel Dungeon, great time sink

I used to play Sauerbraten, a Quake-like FPS, way back in the day. Doesn't look like it's very active any more.

Tux Racer is the OG.

If you enjoyed Zero-K, there is also Beyond All Reason in development and in playable alpha state

Colobot was made open source a while ago and is still great! The game assets (and original binaries) can also be downloaded for free from the dev's site.

Quake and Doom.

I like re3, reVC and reLCS. These are reverse engineered versions of GTA 3, Vice City and Liberty City Stories with improvements. Take Two has thrown a strike on the main repository, but you can still find mirrors or builds in the internet.

I actually love IVAN, and I feel like it has been completely forgotten https://github.com/Attnam/ivan

Widelands is a good game in the style of Settlers 2.

(The later games in The Settlers series moved away from the distribution-network style of gameplay in S2; so Widelands how carries that torch.)