Valve has released the Source SDK which includes Hammer, XSI EXP for Half-Life 2, compiling tools, the Source Model Viewer, documentation on programming, modeling, building materials, and more. This is a precursor to the full SDK which will follow soon after Half-Life 2 is released. Download on Steam.

"Valve has released the Source Software Development Kit (SDK) Tools via Steam. A precursor to the release of the full SDK, which will be released shortly after Half-Life 2 is made available, 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 info from http://www.valve-erc.com/srcsdk/

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