FWIW there is Xash3D[0] which while not technically open source, it does provide some of the practical benefits of having an open source engine (the issue is th

FWIW there is Xash3D[0] which while not technically open source, it does provide some of the practical benefits of having an open source engine (the issue is that Xash3D uses some HLSDL headers and needs to link against HLSDK to work but HLSDK is not compatible with GPL - the devs have tried to contact Valve for letting them use HLSDK with GPL but Valve never responded). Specifically it has been ported to a bunch of platforms and supports not only Half-Life but also other GoldSource-based games (e.g. i was able to play Gunman Chronicles using it - i have the original CD version but it has DRM that makes it impossible to play under Linux, meanwhile with Xash3D the game plays natively and i can use raw mouse input and widescreen resolutions). [0] https://github.com/FWGS/xash3d-fwgs https://github.com/FWGS/xash3d-fwgs