If you are designing forward-facing web sites using the CSS-box model layout, an essential tool for how your model is behaving is the IE Dev toolbar (get it here: http://tinyurl.com/2ddezw). I also check my sites in Firefox which also has a handy tool for developers (get it here: http://tinyurl.com/2z4tvg). These tools are invaluable when you have to design pixel by pixel to make things perfect.
And because I've used it in my post, I might as well tell you about TinyUrl. My developer-in-arms Pablo showed me this site and I use it all the time. If you have a horribly long URL, especially the encrypted versions, and you want to send a link to your peeps (or to your mother), then use this page: http://www.tinyurl.com. We've found that even Microsoft is using this tool in their MSDN pages :)