Asklemmy

OpenRCT2

An open source re-implementation of RollerCoaster Tycoon 2

SuperTuxKart and Mindustry are so much fun!

• OpenTTD
• Net hack
• Boom/Doom
• Quake/Quake 2

The ones that come to mind are Beyond All Reason and Battle For Wesnoth.

OpenRA and OpenTTD

Mobile on Android: Shattered Pixel Dungeon and Techmino.

Space Station 13.

Too many amazing moments and memories to even count, always more to learn. I love how the open source nature of the game means there's many different servers branching off from each other, running their own custom versions of the game. Smaller servers "downstream" pick and choose which features they want to keep when the upstream servers implement something new.

Simon Tatham's Portable Puzzle Collection

It's available on Linux, Windows, Android, iOS, and the web.

I play them on my phone to pass time.

Endless Sky. And AisleRiot Solitaire.

Simutrans and Cataclysm: Dark Days Ahead

it's not really a game, but openmw

Endless Sky for me

Definetly openTTD. I love trains and its so cool to see your little trains drive around, load some cargo, etc.

Xonotic

Cube 2 Sauerbraten, an online FPS. Not many players left anymore though.

Cataclysm: Dark Days Ahead Battle for Wesnoth OpenTTD Doom

I still haven't tried it, but I've heard how great Veloren apparently is. It's an MMO voxel game that takes inspiration from Zelda: Breath of the Wild and is written in Rust.

Open sudoku?

As you can imagine, I'm no fun at parties...

Pixel Dungeon, Battle for Wesnoth

Mindustry, Endless Sky, and Unciv.

Mindustry, SuperTuxKart and Thrive probably

Haven't played it much but for all fans of Thief games there's foss game called The Dark Mod. And I haven't seen it mentioned here, which is a shame.

I am surprised that 0 A.D. is not mentioned.

Although initially unplayable, the game was fun in the mid-to-late 2000s.

I haven't checked it out in a while, but it holds high nostalgic value for me.

FreeCiv is a classic and still fun. rogue and nethack are good, too.

Mindustry!

$10 on Steam, free everywhere else

• Endless Sky -- open-source space game. I actually contributed to it back in the day; a date format option and a full-blown storyline about an author. Unfortunately the storyline is in development hell cause I lost motivation to work on it.
• OpenTTD -- really awesome, with NewGRFs and mods you can have a somewhat "realistic" rail experience (as in, using actual real-life trains. Obviously a pixel game isn't the most "realistic" with graphics)
• Mindustry
• Pioneer Space Sim

Super Tux kart I think it's underrated and fun

Unciv is pretty cool, it is Civ 5 but you can play on mobile as well. It was great to have the "One more turn" feel while stuck on a train.

DCSS, Minetest and Simon Tatham's Puzzles.

supertuxkart, mindustry, unciv and xonotic are my favorites so far

Xonotic.

Formerly known as Nexuiz, until the lead dev sold the naming rights.

Widelands is a great strategy / building game. The gameplay and UI style is a niche - but that's one of the things I like about it. It's doing something different to most games.

(The gameplay is similar to Settlers 2; before that franchise changed direction.)

[edit] But the open-source game I've spent the most time playing would be OpenXcom-extended, with xpiratez. That game is truly huge.

Since no one mentioned yet, Tales of Maj'Eyal deserves some love too. One of the best roguelikes out there..

Veloren!

Haven't seen this one mentioned but I have fond memories of Frets on Fire which is a guitar hero clone

Xonotic (quake-esque FPS, IMO its like quake and halo had a FOSS child), Minetest (a voxel game engine, multiple games are available for it), and Mindustry (sandbox tower defense) are the only ones ive tried so far

vim

Velorem and Endless Sky!

Osu. Please save me.

VCMI - it's re-written from scratch, open source, multiplatform Heroes 3 engine with many improvements and mods manager. It requires some files from the original game though.

Taisei and Minetest with VoxelLibre