You will notice "tags" in front of each title like [W] or [AL] or [ALW]. Those are the platforms the game is compatible with be it "official" or through wine/proton which are like this:

• Games that run natively on linux won't have the Proton tag. (Though all open source games I found so far run natively on linux)
• Games with Proton support will have a link to ProtonDB so you can check it's compatibility.

• Arcade
• Card
• Rythm
• FPS
• Horror
• MMO
• Puzzle
• Racing
• RPG
• Sandbox
• Side scroller
• Simulation
• Strategy

• Bastet [?] - Evil falling block game.
• Destination SOL [ALMW] - Pilot your ship in the seamless open world made of space, planets, and more. Find new ships and equipment, find allies and beat all the enemies in this open source space shooter!
• fooBillard [ALW] - A free (libre; licensed under GPL2) OpenGL billiard game for Linux by Florian Berger.
• Leblebi Games' Tetris [A] - Unity with C# 3D Tetris Game Development.
• VGA Golf [LMW] - VGA Golf is a local or networked multiplayer golfing sim, modeled on 90s era console and DOS games (hence the term VGA: Video Graphics Array).

• Arcmage [ [IRL] ] - A libre card game.

• AssaultCube [LMW] - AssaultCube is a FREE, multiplayer, first-person shooter game, based on the CUBE engine.
• Cube [LMW] - Cube is an open source multiplayer and singleplayer first person shooter game built on an entirely new and very unconventional engine.
• Cube 2: Sauerbraten [LMW] - Cube 2: Sauerbraten is a free multiplayer & singleplayer first person shooter, the successor of the Cube FPS.
• DOOMRetro [W] - The classic, refined DOOM source port. For Windows PC.
• Red Eclipse [LMW] - Free and Open Source game, built on Cube Engine 2 using SDL and OpenGL which allows it to be ported to many platforms.
• Tesseract [LMW] - Tesseract is a first-person shooter game focused on instagib deathmatch and capture-the-flag gameplay as well as cooperative in-game map editing.
• Urban Terror [LMW X] - Urban Terror™ is a free multiplayer first person shooter developed by FrozenSand, that will run on any Quake III Arena compatible engine. Only engine is open source.

• Penumbra Overture [LMW]- Penumbra: Overture is a first person survival horror game, emphasizing on survival and problem solving instead of mindless violence and gore.

• Daimonin [LW] - Daimonin is an open source project and a game engine.
• Manaplus [ALMW] - ManaPlus is a free OpenSource 2D MMORPG client, supports hercules, tmwAthena, evol.
• PlaneShift Legacy[LMW X] - PlaneShift Legacy is Open Source for the client and server code, this version is still playable but devs are no longer expanding on it.
• Ryzom [LMW] - Ryzom is one of only a few commercial-grade MMORPGs that are fully open source: client, server, tools, and media.
• Veloren [LMW] - Veloren is a multiplayer voxel RPG written in Rust. It is inspired by games such as Cube World, Legend of Zelda: Breath of the Wild, Dwarf Fortress and Minecraft.

• aaaaxy [ALMW] - AAAAXY is a nonlinear 2D puzzle platformer taking place in impossible spaces.
• Enigma [LMW] - Enigma is a puzzle game inspired by Oxyd on the Atari ST and Rock'n'Roll on the Amiga.
• Pingus [AHLMW *] - Pingus is a free Lemmings^tm-like puzzle game covered under the GNU GPL.
• Pipeanic [L *] - Pipepanic is a pipe connecting game using libSDL. Connect as many different shaped pipes together as possible within the time given. Supports a lot of obscure platforms.

• Armagetron ADVANCED [LMW] - "A Tron clone in 3D."
• GL Tron [A] - GLtron is a 3D snake game based on the light cycle portion of the film Tron.
• SuperTuxKart [ALMNW] - SuperTuxKart is a 3D open-source arcade racer with a variety characters, tracks, and modes to play.
• Tux Racer [LMW] - Tux Racer lets you take on the role of Tux the Linux Penguin as he races down steep, snow-covered mountains.
• Tux Rider [A] - A port of well known ‘Tux Racer’ 3D game for Android.

• POWDER [LMW *]- A Roguelike designed for the Gameboy Advance and Nintendo DS.
• /tg/station 13 [LW] - Space Station 13 is a two dimensional, multiplayer, role playing game developed on BYOND.

• StepMania [LMW] - StepMania is a free and open source, cross-platform rhythm game. It supports common key-based rhythm game formats (including 4-panel and 5-panel dance games among others), as well as keyboard and dance pad controller.
• Performous [LMW] - An open-source music and rhythm game.

• Malefactor [B] - Malefacotr is a strategy text game in which the player takes on the role of a Sauron-style Lord of Darkness with the goal of conquering the world.
• The Battle for Wesnoth[ALMW] - The Battle for Wesnoth is a free and open-source turn-based strategy video game with a high fantasy setting.
• TripleA[LMW] - The World's Foremost Open Source Grand Strategy Game.

• runfoxrun [A] - An attempt to make a complete game for Android based devices within 14 days.

• EmptyEpsilon [ALW] - EmptyEpsilon places you in the roles of a spaceship's bridge officers, like those seen in Star Trek. While you can play EmptyEpsilon alone or with friends, the best experience involves 6 players working together on each ship.

• OpenTTD [ALMW] - OpenTTD is an open source simulation game based upon the popular Microprose game "Transport Tycoon Deluxe", written by Chris Sawyer. It attempts to mimic the original game as closely as possible while extending it with new features.

• Thrive ¹ [W] - Thrive is a free, open-source game about the evolution of life.

• YSFligtht ^{1 2} [LMW X] - According to dev open source is less future proof than closed source ¯_(ツ)_/¯.