In today's Tech Times newsletter from SitePoint, there was a small section about these things called bookmarklets, and a main article discussing them.
http://www.sitepoint.com/article/bookmarklets
They're bookmark links with embedded Javascript, and upon accessing them they render the current page with their useful functions.
Some of the ones I've seen are sort of just for fun, but the two given in the newletter are simply unbelievable. They'll make troubleshooting web design much easier.
Topographic Page Layout
Mouseover DOM inspector.
Go to the pages, and just add the bookmarketlet as a bookmarket. Older browsers might suffer from using them, but if you have the latest IE or Mozilla (w/javascript support), you should have no problem.