Probably any experienced web developer would be familiar with this problem: over time your css files can grow pretty huge and ugly because of all the no longer used selectors, w
Check out uCSS library from Opera Software.
It helps you to find unused CSS, as well as duplicate CSS. Also, you can get an overview of how many times each rule has been used in your markup. Several options are available by setting up a config file.
Update:
Another great alternative: csscss.
Written in Ruby and supports SASS, Less.
Update:
Another great alternative: uncss.
It works across multiple files and supports Javascript-injected CSS.