This post was published more than a few years ago (on 2008-01-04) and may contain inaccurate technical information, outmoded thoughts, or cringe takes. Proceed at your own risk.
If you're a website designer (or just tweaking your WordPress CSS layout, Browsershots is invaluable. It shows you how your site looks in dozens of browsers, and you can get it free, or pay for speedier access. Go see this site in Browsershots for an idea of how useful it is (and how broken my site is).