Phantomjs - Headless WebKit with JavaScript API

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. It is an optimal solution for headless website testing. It run functional tests with frameworks such as Jasmine, QUnit, Mocha, Capybara, WebDriver, and many others.
It is capable to do screen capture. It could create web site screenshots with thumbnail preview. Automates performance analysis using YSlow and Jenkins.