Open Source Games, Projects, and Tools

Tools

git: The ubiquitous version control.
gdb: Segfaults begone!
RenderDoc: The holy grail of graphics debugging. Cross-platform and works for multiple graphics API. If you are learning or using a graphics API, you are only hurting yourself by not taking advantage of this tool.
Shadertoy: A good website to practice shaders (think of it as a shader sandbox). The webclient uses GLSL ES, so it's easy to translate what you've made into a program that uses OpenGL or Vulkan flavours of GLSL.
Tiled: A popular tile map editor.

Opportunities to learn from the source code.

0 A.D.: Historical RTS that started out as a mod for Age of Empires II: The Age of Kings.
Amphibia: Calamity Beatdown: Fan-made side-scroller for the Disney animated series Amphibia. Source code here.
Brogue: Roguelike game.
Cataclysm: Dark Days Ahead: Ambitious post-apocalyptic survival roguelike.
CnC Remastered Collection: Open sourced 2020 remaster of Command & Conquer (1995) and Red Alert (1996).
Mindustry: TD/RTS factory management.
Minetest: Pretty much an open-source Minecraft with some differences.
OpenGOAL: Native port of Jak and Daxter to PC which involved decompiling the original GOAL code and developing a GOAL compiler.
OpenLara: Open source recreation of Tomb Raider 1 to 5.
OpenRCT2: Open source recreation of RollerCoaster Tycoon .
Quake III Arena: FPS famous for popularizing an old bit hack trick .
RuneLite: Popular Old School RuneScape client.
SuperTuxKart: Racing game that is similar to Mario Kart.
Tales of Maj'Eyal: Age of Ascendancy: ToME is a rouguelike with an active community and a dash of modern flair.
Wesnoth: A turn-based strategy game in a fantasy setting. There are multiple campaigns to play through and you can create maps to share with friends.