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

Dear ImGui: Bloat-free Graphical User interface for C++ with minimal dependencies

Compatibility tool for Steam Play based on Wine and additional components

Gaming meets modern C++ - a fast and reliable entity component system (ECS) and much more

OpenMW is an open-source open-world RPG game engine that supports playing Morrowind. Main repo and issue tracker can be found here: https://gitlab.com/OpenMW/openmw/

Sol3 (sol2 v3.0) - a C++ <-> Lua API wrapper with advanced features and top notch performance - is here, and it's great! Documentation:

Qt Node Editor. Dataflow programming framework

Cista is a simple, high-performance, zero-copy C++ serialization & reflection library.

HL2-style, classic FPS movement for Unreal Engine implemented in C++

A modern C++ tweening library

Qt Code Editor widget.

Cortex Command - Open Source under GNU AGPL v3 (no game data included)

Render Pipeline Shaders SDK

gigantic, obese SourceMod extension library of sigsegv's TF2 mods (mostly MvM related)