o3n CMS Importer is a tool to import articles / posts between CMSs. It is an Adobe AIR application written in Flex 4.6. It currently supports imports between Joomla 2.5 and Wordpress 3.4.1 (other versions not tested)
It only copies the articles not categories, authors, meta keywords, tags etc. It requires for you to go over the articles to set those information right.