Xash3D FWGS Engine 
Latest version of the engine is available at: https://github.com/FWGS/xash3d
Orignal project: Xash3D on ModDB
Xash3D Engine is a custom Gold Source engine rewritten from scratch. Xash3D is compatible with many of the Gold Source games and mods and should be able to run almost any existing singleplayer Half-Life mod without a hitch. The multiplayer part is not yet completed, multiplayer mods should work just fine, but bear in mind that some features may not work at all or work not exactly the way they do in Gold Source Engine.
Xash3D FWGS is a fork of Xash3D Engine, which aims on crossplatform and compatibility with original Xash3D and Gold Source.
License
The library is licensed under GPLv3 license, see COPYING for details. CMakeLists.txt files are licensed under MIT license, you will find it's text in every CMakeLists.txt header.