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.
Seed is a library and interpreter, dynamically bridging (through GObjectIntrospection) the WebKit JavaScriptCore engine, with the GObject type system. In a more concrete sense, Seed enables you to immediately write applications around a significant portion of the GNOME platform, and easily embed JavaScript as a scripting-language in your GObject library.
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.
XRAY is a bookmarklet for Internet Explorer 6+, and Webkit and Mozilla based browsers (including Safari, Firefox, Camino or Mozilla). Use it to see the box model for any element on any web page.