Openage

Open-source engine for Heroes of Might and Magic III

C++20 framework for creative coding / Cross-platform support (Windows, macOS, Linux, and the Web)

Space exploration, trading, and combat game.

Widelands is a free, open source real-time strategy game with singleplayer campaigns and a multiplayer mode.

An open source re-implementation of Chris Sawyer's Locomotion

GemRB is a portable open-source implementation of Bioware’s Infinity Engine.