Want specific web testing tools? Try this list of 550+. Great if you already know roughly what you want to test. A little complicated if you're just trying to make sure your site looks right to all users.
To cut through this jungle of testling tools, or rather to encapsulate all levels of testing into one single tool, Kevin Menard developed Mogotest, a 'web consistency' service (his own coinage) designed to automate, at the level of user experience, every kind of web testing imaginable.
He explains the idea in greater detail, but here's the basic idea, in his own words:
Until now, our choices have been ship & hope nothing breaks or compare every browser rendering and see if anything gets caught. I.e., our choices have been somewhere between not testing and exhaustive manual testing, which is a gigantic waste of human talent. Web Consistency Testing is all about automating how your users visually interact with your site. It addresses a massive gap in our Web testing process while putting emphasis back on the user.
That's a little abstract, but fortunately Kevin also gave a detailed presentation at the recent Google Test Automation Conference in Mountain View, CA:
The talk gets pretty theoretical in places -- but testing for user experience can be a pretty fuzzy thing, so a conceptual overview can definitely help.
Whether or not you're sold on Mogotest in particular, a unified approach to web testing will definitely be a good thing.