Adobe Browserlab is a Flash cloud based application that competes with Microsoft SuperPreview. Basically, it allows you to test your web site and see how well it works on most popular web browsers. At first, you will have to sign up for the preview (registrations are closed at this moment). Once signed in, you will be able to choose browsers, operating systems to test and see them side-by-side.
Of course, you can always BrowserShots.org if screenshot is more than enough.