Run IE8/IE7/IE6, Firefox, Safari, Chrome, and Opera from the web

Haven't tried this yet but it might be useful.

Run IE8/IE7/IE6, Firefox, Safari, Chrome, and Opera from the web: "The Xenocode Browser Sandbox allows all popular Windows browsers to be run simultaneously, directly from the web. Web designers, system administrators, and other users can now evaluate Internet Explorer 8, 7, and 6, Mozilla Firefox, Apple Safari, Opera, and Google Chrome directly from the web in a risk-free manner."

Update: I tried their IE6 sandbox and it worked great.

Chickenfoot

Chickenfoot: "Chickenfoot is a Firefox extension that puts a programming environment in the browser's sidebar so you can write scripts to manipulate web pages and automate web browsing. In Chickenfoot, scripts are written in a superset of Javascript that includes special functions specific to web tasks."