Blender

Blender is an open source (free) 3D Modeling package that can be used to create static and physics 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.

Plugins

• .smd Exporter by Pelle Johnsen (that's a dead link, use http://files.johnsto.co.uk/smd_export.py instead)
• .smd Exporter - work in progress Check this thread for updates. Made by Dragonlord.

Note:Both exporters do not have full support of all features!

• .smd Importer by 3D2Toy.

See also

Is Blender 3D Good for Source/How to Start

Modeling props with Blender

Official Website

Blender 3D: Noob to Pro wiki