WhatFont is the easiest way to inspect fonts in a webpage. Firebug or Webkit Inspector? No, that’s too complicated. It should be just a click away. Once pressed, it displays the font used for any text hovered.
Hence I wrote WhatFont, with which you can easily get font information about the text you are hovering on.

If you click on the text, it drops a pin with the details of the font including style, weight, line-height and font-size. These pins are not removed after each click, so, multiple items can be easily compared.
Also, WhatFont detects fonts served from services like Typekit or Google Font API and displays this info.
Simply, a must-use for every web designer.
Compatibility: All Modern Browsers
Website: http://chengyinliu.com/whatfont.html
Download: https://github.com/willow/WhatFont-Bookmarklet