The advent calendar tutorials are still going on, and the two latest tutorials explain the way to implement tags in symfony.
Whether you want to add tags to a post (with a cute AJAX autocomplete form) or see the popular tags for another one, all it takes is a few lines of code to do it with symfony. A tag cloud is also developed in minutes, so you will have no excuse to sort things in tree-structured categories again.
As usual, the source code is explained in detail and available for download.