How does your (public) application look in [insert-browser-and-version] on [insert-os-and-version]?
Testing web applications is a tedious job. Gazillion combinations of browsers and operating systems can turn that into a nightmare (suddenly a rich client doesn't look that bad anymore). Scott Hanselman has a very good blog post on testing considerations. One of the services he introduces in the post is Browsershots that allows you to take screenshots of your site using many combinations of browsers and operating systems. The only catch: the site must be publicly accessible and the free service will publish your images. There is a private service available for a fee. Neat tooling for visuals. A similar offering is available from MultiBrowserViewer which requires a client (Win only) installation. Still you need to test functionality and performance. Here you might look at JMeter for performance or Selenium for functional tests. Not to forget HTTPUnit, XPages Unit Tests and of course IBM's commercial tools like Rational Functional Tester and Rational performance Tester (For testing Notes client applications you would use Smart Toucan's AutoUser). Of course performance testing comes with its own set of caveats (Don't bother to test what you can't fix).