Blender

Blender is an open source (free) 3D Modeling package that can be used to create static props. Although XSI is the Valve-recommended program for modeling, some will find Blender to have an easier learning curve. The free version of XSI for Source modeling has some limitations and watermarks images.

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See also

Learning Blender (good first steps)

Modeling props with Blender

Official Website

Blender 3D: Noob to Pro wiki