XRAY is a bookmarklet for Internet Explorer 6+, Webkit and Mozilla based browsers (including Safari, Firefox, Camino or Mozilla). Use it to see the box model for any element on any web page.

I came across this great little plugin courtesy of Stylegala. Although it seems like a useful addition to my browser, im not sure it will replace CSSEdit, which has been an indispensable tool in helping me get to grips with CSS.