Here's one I'd stumbled across a couple of years back now and had subsequently forgotten about but Lord knows why because it's so useful and so simple in it's conception. Google Labs has a Browser Size tool that let's you overlay a summary of browser size (based on visits to the Google homepage) over any web page. For a while people had spoken about there being no page fold when it came to web design. Well that's poppycock, if multivariate testing and UX testing has taught us anything it's that if you have content which people have to scroll down to a lot of people will either not bother or just not realize the content is there in the first place. In testing I've found that if you cant get away from a lengthy page you need to make the design imply that its worth scrolling to the content or try and bring everything back above the fold through tabbed design etc.