CasperJS!

Selenium is a suite of tools specifically for testing web applications. Selenium IDE is a Firefox add-on that records clicks, typing, and other actions to make a test, which you can play back in the browser. Selenium Remote Control (RC) runs your tests in multiple browsers and platforms. Tweak your tests in your preferred language.

PhantomJS is a headless WebKit with JavaScript API. It has fast and native support for various web standards: DOM handling, CSS selector, JSON, Canvas, and SVG.

SlimerJS is a scriptable browser. It allows you to manipulate a web page with a Javascript script: opening a webpage, clicking on links, modifying the content... It is useful to do functional tests, page automaton, network monitoring, screen capture etc.

The httest provides a large variety of HTTP-related functionality, useful for implementing all kinds of HTTP-based tests.

A lightweight Python library that uses Webkit to enable easy scraping of dynamic, Javascript-heavy web pages

DalekJS is an open source UI testing tool.

PhearJS renders dynamic webpages using PhantomJS: fetch a page, render it and return a pretty JSON object. PhearJS is offered as a hosted API as well as an open source solution that you can host yourself.

Screenster provides visual UI test automation for web applications. It is the only tool that validates the screens that users actually see.

A scripting tool for browser automation. By executing user given instructions contained in a javascript "TRON" file it automatically browses web, clicks on links, fills out forms, etc.

WebInject is a free tool for automated testing of web applications and web services. It can be used to test individual system components with HTTP interfaces, as a test harness to create a suite of functional and regression tests, or for service-level monitoring.