This is not the same thing as the SDK, but still something that'll be incredibly useful for aspiring mod developers.
The SoftImage|XSI EXP is a complete version of SOFTIMAGE|XSI that lets you output Half-Life 2 characters, props and worlds. The XSI EXP for Half-Life 2 has all the tools you need to make your very own artwork for the world's coolest game, from polygon modeling and lightning fast subdivision surfaces, to nonlinear animation mixing and interactive simulation and rendering.
Information and download by PlanetHalflife and FilePlanet