I know of some extensions implementing ads, please have a look at the list on http://ezpedia.org/wiki/en/ez/advertising. AFAIK eZ publish has tagging possibilities with the new Website Interface of 3.9 and there are extensions also implementing labeling ( http://ez.no/community/contribs/applications/ezlabel ), tag clouds ( http://ezpedia.org/wiki/en/ez/tag_cloud ) and voting/rating systems ( http://ezpedia.org/wiki/en/ez/rating ).
