I'm a big fan of Symfony. It is the framework behind Delicious, Yahoo! Answers, Yahoo! Bookmarks. I also used it to create <a href="http://www.w3counter.com" rel="nofollow">http://www.w3counter.com</a> and <a href="http://www.w3roi.com" rel="nofollow">http://www.w3roi.com</a>.