OpenXRay is an improved cross-platform version of the X-Ray Engine, which is the original game engine used in the S.T.A.L.K.E.R. game series by GSC Game World. However, it seems a bad actor got it taken down from GitHub.
Update - 11/01/2024, 11:55 AM UTC: The GitHub repository has been restored now. Hopefully this won’t happen in future, considering as noted below, GSC Game World actually gave it their blessing to be used. And thanks to the take-down, even more people now know about it. From the reversal notice, GitHub stated clearly after a review it was a fraudulent take-down.