Godot Engine – Multi-platform 2D and 3D game engine

Simple and Fast Multimedia Library

Industry-standard navigation-mesh toolset for games

C++ bindings for the Godot script API

Orchestrator: Unleashing Creativity with Visual Scripting

Origin Repository for SCP FreeSpace 2 Open

SFML Extension with various game programming features, like particles, animations, vector operations

The source code for the Menge crowd simulation framework

Simple and Fast Network Utility Library

Header-only library with unconventional C++ features, like deep-copy smart pointers, dynamic dispatch, type-erased ranges

Godot Engine – Multi-platform 2D and 3D game engine

LÖVE is an awesome 2D game framework for Lua.