defining & ordering browsing navigation steps
filling & submitting forms
clicking & following links
capturing screenshots of a page (or part of it)
testing remote DOM
downloading resources, including binary ones
writing functional test suites, saving results as JUnit XML
scraping Web contents
SlimerJS is useful to do functional tests, page automation, network monitoring, screen capture, etc.
SlimerJS is similar to PhantomJs, except that it runs Gecko, the browser engine of Mozilla Firefox, instead of Webkit (And it is not yet truly headless).