PhantomJS : PhantomJS is a headless WebKit with JavaScript API. It has fast and native support for various web standards: DOM handling, CSS selector, JSON, Canvas...
google-mobwrite : MobWrite converts forms and web applications into collaborative environments. Create a simple single-user system, add one line of JavaScript, and inst...
domsnitch : A passive reconnaissance tool inside the DOM (experimental)
JS Bin is a tool for experimenting with web languages. In particular HTML, CSS and JavaScript, but JS Bin also supports other languages too (like Markdown, Jade and Sass).
JS Bin is ultimately a code sharing site.