ProCSSor is a powerful (and wholly free) CSS prettifier that lets you format CSS in the exact way you want. It empowers you to turn your CSS into something that is visually more compelling, and with a minimum of effort at that.
A web crawler follows the links of your site. On each page, we retrieve the CSS files and list the selectors. The selectors are tested on the html page. If the selector matches an element, it is marked as used. When all the files have been visited, a new css file with only the used selectors is built. That’s the file you can download.
» Read more: The most efficient and newest CSS3 tools