PhantomJS is a headless WebKit scriptable with a JavaScript API. It has fast and native support for various web standards: DOM handling, CSS selector, JSON, Canvas, and SVG.
SlimerJS is useful to do functional tests, page automation, network monitoring, screen capture, etc.
SlimerJS is similar to PhantomJs, except that it runs on top of Gecko, the browser engine of Mozilla Firefox (specifically, version 31), instead of Webkit, and is not yet truly headless.