It is part of the free Excitech Revit Toolkit. It has also been around for a long time, as Revit add-ins go. The other key function of this tookit is the Element Renumber tool.
Download by clicking on appropriate version link at the bottom of this page. You will need to register / login.
Revit @ Waterman: Schedule those Levels - A Method (with a little help from our friends)