OpenSAGE is a free, open source re-implementation of SAGE, the 3D real-time strategy engine used in Command & Conquer™: Generals and other RTS titles from EA Pacific.
The engine is under active development by a small group of volunteers. While the engine doesn't yet implement much of proper gameplay for any of the supported games, it can already load most types of assets from the first few SAGE titles, load into maps and display user interfaces.
The engine is written in modern C# and runs on Windows, macOS and Linux using .NET Core. We use Veldrid in order to support a variety of graphics APIs (including Direct3D 11, OpenGL and Metal) with a single codebase. The source code is available on GitHub under the LGPLv3 license.