Drush tool to dump/update CTools (and some other) exportables.
To a site developer, CTools exportables (which includes Panels, Page Manager pages and Strongarm variables) and default Views, is a godsend for creating sites and then storing the setup in code. However, exporting the objects and saving them in the right files is a bit of a chore, and it is here Drush EM helps out by providing some simple commands.
In a way it's an alternative to using the Features module, but which leaves package and release management up to the developer.
Due to the current status and nature of CTools, Views and other modules Drush EM supports, it currently contains quite a bit of module specific code. Hopefully things will stabilise, so new modules should be supported out of the box.