Game engines

Production-quality:

• Godot
• Unreal
• Unity
• https://github.com/ConfettiFX/The-Forge (apparently used by Bethesda & others)
• https://github.com/mosra/magnum (more for visualization)

Toy:

• https://github.com/crownengine/crown (editor looks nice)
• https://github.com/volcoma/EtherealEngine

Unsorted:

• Minimalist! https://github.com/raysan5/raylib
• https://wiki.ryzom.dev/
• https://github.com/defold/defold/
• https://bevyengine.org/
• https://github.com/clibequilibrium/EquilibriumEngine
• FlaxEngine

Renderers:

• https://github.com/google/filament
• https://github.com/Themaister/Granite

Graphics libraries:

• https://github.com/DiligentGraphics/DiligentEngine

Texture unwrapping:

• https://github.com/jpcy/xatlas