Ajaxian is reporting about a very neat and cool bookmarklet, XRay, that helps you visually get information about specific elements on your page. The site best describes its functionality as “a free cross browser tool (a bookmarklet) that lets you see the box model in action for any element, letting you see beneath the skin of any web page. Just click the XRAY button to instantly answer those vital questions: where is the top and left of this element? how big is each margin? how big is the padding? how wide and high is the content box?”.
I tried this out and it was VERY cool. While there's some overlap with Firebug, this is definitely being added to my toolkit. The only downside is that it doesn't work on IE.
