I just released my first public Chrome Extension: unusedCSS. You can download it on the Chrome WebStore.
I know that tools doing that are already existing. But these tools are not targeting the same kind of site. They target content website where the page is generated on the server and the user is navigating from page to page.
This extension is reaction to navigation event and DOM modification event and is trying to react to these event to update the list of used selector.
It is working quite well, with good performance, but some love needs to be apply: I need a logo, improve the results, try to do a crawling option, move the tool in the developer tools, etc
The source are available on Github, let me know if you have ideas to improve it, or found a bug.