A Phoronix reader wrote in this morning to bring up Xash3D, which is self-described as a "Custom Gold Source Engine build from a scratch." The Xash3D engine isn't brand new but has been around for a while now and is continuing to mature -- it should already work for almost any existing single-player Half-Life mod without having any compatibility issues. The multi-player code to the Xash3D engine is also incomplete at the moment.
While retaining compatibility with the original Valve Gold Source Engine, at the same time this new Xash3D engine ups the limits for handling greater 3D, sound, and other assets. Besides upping various limits, there's realistic lighting values on server, improved decal save, entity patch technology, support for various maps, hot resource pre-caching, light-style interpolation, improved model and sprite lighting, and many other features.
The Xash3D engine is similar to similar open-source initiatives for re-implementing game engines as open-source to support assets/games from other companies, like the open-source BioWare games.
The Gold Source Engine, a.k.a. GoldSrc, was the original name for Valve's game engine that was a heavily-modified Quake Engine code-base that powered Half-Life. Gold Source was succeeded by the Source Engine, but not before powering many other post-HL titles like Team Fortress Classic, Counter-Strike, Day of Defeat, and the original Counter-Strike off-shoots.
The latest release of the Xash3D engine is build 2015 and was released back in mid-August while introducing many new features. Xash3D was originally introduced in April of 2011.
More information on the project is available from its ModDB.com page. At the moment the Xash3D engine is primarily targeting Windows support, but with it being GPL-licensed, interested developers could bring the support to Linux. Below is a video demo of Xash3D.