Wall Worm is a set of pipeline tools for 3ds Max and the Source Engine. It allows the artist and development team to create models, textures, materials, levels and more inside 3ds Max. It was originally known for the Wall Worm Model Tools that optimizes the model-exporting process, but has expanded into the entire Source pipeline.
You can learn more using 3ds Max as your level editor by reading Hammered to the Max: A Hammer User's Guide to 3ds Max.
You can get Wall Worm at http://dev.wallworm.com/