<a href="http://seleniumhq.org/" rel="nofollow">http://seleniumhq.org/</a> is great for system testing. Easy to start using with the firefox plugin (selenium IDE), more advanced features (Selenium RC, Grid) and tests can be written in a number of languages.
<a href="http://browsershots.org/" rel="nofollow">http://browsershots.org/</a> is useful for checking layout on every browser/os combination imaginable.