Rated 4 out of 5 stars
As far as I know, it's the only add-on of its kind. Its automatic validation approach allows to detect that elusive unclosed tag that only shows up when you've loaded certain data (thus goes unnoticed with manual validation). Definitively, a must-have for webmasters.
The only major drawback is that it doesn't support HTML5 (it only provides a link to submit code to an online validator and such feature is provided by other extensions that do it better).
BTW, make sure you only use it on your own sites. There's no point in wasting memory and CPU cycles validating HTML at Google or Yahoo.