From the Wordpress codex:
Joomla
Joomla to Wordpress Migration - Web service to migrate Joomla 1.5, 1.6, 1.7, 2.5 and 3.0 to Wordpress 3. Compatible with K2
Joomla 1.5 Importer - Plugin for Wordpress
Joomla to Wordpress Import Wizard
Joomla to Wordpress Migration Wizard
FG Joomla to WordPress...