As previously mentioned, the Web Developer extension for Firefox is freaking awesome. I'm awed (again) by it's wonderfulness. You can outline your CSS elements to see who's got the freaky-large border, view your print and handheld stylesheets, and even disable CSS altogether to check for web standards / accessibilty issues. And that's only one little menu! It makes me feel like this. See the Penny Arcade Journal for info on the image.