From Adobe's website:
BrowserLab provides web designers exact renderings of their web pages in multiple browsers and operating systems, on demand. BrowserLab is a powerful solution for cross-browser compatibility testing, featuring multiple viewing and comparison tools, as well as customizable preferences. Since BrowserLab is an online service, it can be accessed from virtually any computer connected to the web. Also, Adobe Dreamweaver® CS4 software users have access to additional functionality such as testing local and active content.
Great little testing tool, that took a lot of work I'm sure!
View the Browser Lab Website
(FYI: i was able to login with my Adobe credentials that I use for forums, Alpha stuff, etc)
Some gripes... there is no choice for Chrome, which I realize is new-ish, but come on guys :) Also, no choice for Mobile Browsers? I guess these things will be coming soon enough!