The SketchUp Source Tools allow you to create 3D content for the Source game engine using Google SketchUp. There are two main file format exporters: one for .VMF and one for .SMD.
The .VMF export plugin allows you to export SketchUp models to Hammer, the environment editor for the Source game engine. Hammer is used to create game levels for titles such as Half Life, Portal, Counter Strike, Day of Defeat, and Left 4 Dead.
he .SMD export plugin is useful for creating models for props or for making static models that can be placed in your levels.
Visit the Valve Developer Community for information.
[ via Anonymous Commenter ]