Wall Worm Sky Writer is a free 3DS Max script that lets you render out your 3ds Max scene into HDR images for use as sky textures in Source. It can compile the images directly into Source; it also creates a cube cross that is ready for HDR Workshop.
Sky Writer does not design your sky... it simply renders the scene and exports/compiles everything into the right formats and creates VMTs and VTFs. For designing the environment, use the standard 3ds Max tools or use third-party plugins like Dreamscape or Vue.
Now includes a native PFM Bitmap exporter for the HDR files.