Valve have released the Source Software Development Kit via Steam.
This SDK is a precursor to the full Half-Life 2 SDK that will be released shortly after the game is released on 16 November.
... the Source SDK Tools release offers a comprehensive toolset for starting production on Source-based MODs, including Hammer, XSI EXP for Half-Life 2, compiling tools, the Source Model Viewer, documentation on programming, modeling, building materials, and more.
More information is available here and hereDiscussion >>
