It's a plugin for 3ds Max that I use sometimes when I quickly want to export models. I'm pretty sure it's mentioned a couple of times in the tutorials section, as well as the Valve Developer wiki.
In my opinion, the Wallworm tools are fine for quickly getting stuff exported, but I always end up manually editing QC files for more advanced options. It's not as great as the Blender SMD tools, but WallWorm is more focussed on level design for Source in 3ds Max, while the Blender tools are all about integration into the Source asset pipeline. Wallworm also relies on a bunch of (paid) third party tools as well as existing SMD and VTA plugins, so the only real use I get out of it is like I said, quickly generating QC files and exporting with a single click.