Dom Inspector

Extremely useful component of Mo Zilla which allows you to see exactly which bit of the rendered page is made by which Html Element

http://grayrest.com/moz/evangelism/tutorials/dominspectortutorial.shtml (BTW, that page has some very cool layout)
- Sadly no longer available, even on archive.org. :-(
http://www.brownhen.com/DI.html is another introduction.