Extension: http://extensions.joomla.org/component/option,com_mtree/task,viewlink/link_id,495/Itemid,35/ Demo:http://phlooid.com/
yes... probably the mostly used free one is: http://mambo-extensions.com/component/option,com_mtree/task,viewlink/link_id,142/ but a good commercial one is: http://www.phil-taylor.com/Joomla/Components/Tags/
there is only one commercial tag component for joomla which is similar ultimate tag worr. http://www.phil-taylor.com/Joomla/Components/Tags/ . i think it does not worth to buy