Joomla2wordpress is a project mainly written in PHP, it's free.
This script allows a raw wordpress installation to get it's data from a Joomla Database or, in other words, migrate a Joomla to a Wordpress
This script allows a wordpress database to get the articles, categories, users, etc. from a joomla database.
Use:
Truncate the following tables from wordpress: wp_comments wp_posts wp_terms
wp_term_taxonomy wp_term_relationships wp_users wp_usermeta
wp_links
And then execute the script to migrate all the data.