Live.js, once included to a web page, auto-refreshes it automatically if there is an update to the files used. Say goodbye to Alt-Tab. Live.js makes sure you’re always looking at the latest version of the page you’re working on, whether you’re writing HTML, CSS or Javascript.
How its Works:
Just include Live.js and it will monitor the current page including local CSS and Javascript by sending consecutive HEAD requests to the server. Changes to CSS will be applied dynamically and HTML or Javascript changes will reload the page.
Updates to the CSS are applied instantly/dynamically and the ones for HTML + JS will refresh the page.

At the back-end, the script sends consecutive HEAD requests to the server and detects the updates with the help of the responses.
P.S. The same team also has a Firebug extension named Backfire that does a similar job for CSS files being edited via Firebug..
Requirements: No Requirements
Compatibility: All Major Browsers
Website: http://livejs.com/
Demo: http://livejs.com/demo