UltraHLE Source Code 1.0.0
By: Epsilon (95%) and RealityMan (5%)
Get It Here
This will not compile out of the box. You need Microsoft Visual C++ 6.0.
Source code (or docs) for the Glide library, X-Engine, is not included.
Someone has already ported this to use OpenGL. Perhaps they will release it soon.
Look at ultra.h for various info