http://www.culturedcode.com/xyle/

Using Xylescope is like surfing with an x-ray vision. You will immediately see beneath the surface of the web pages you are visiting. Experiment by changing CSS values in author style sheets - no matter if they are on the internet or on your own computer. Export interesting source code with ease and apply new and consistent formatting to your own CSS files. Download the trial version and try it out.

Read our review here