Half-Life Updated is an updated version of the Half-Life SDK that can be used with newer versions of Visual Studio and that includes both bug fixes added to the game but not the official SDK, and bug fixes not yet added to the game.
You can find the changelog here.
You can find the Updated source code here: https://github.com/Solokiller/halflife-updated
There is also a version that uses CMake, which additionally configures the project to deploy dlls to the mod directory and sets up debugging parameters: https://github.com/Solokiller/halflife-updated-cmake
The old thread about this lists other changes: https://twhl.info/thread/view/19592