We have just added a cool bookmarklet that makes our site even easier to use for website compatibility testing. You can find it on the My Testing Center.
Once you install the bookmarklet, you can click on it to live test any site you are currently on!
What is really cool about the whole thing is how it can change your workflow in testing. You can work on a site and get it to your liking and two clicks later you can look at that site in IE6, Opera or Chrome. You can even run screenshots.