http://forge.joomla.org/sf/projects/accessible_joomla

Accessible Joomla! is an amazing rewrite of the entire Joomla! CMS system to make it 508 compatible as well as many other additional accessibility additions such as additional classes and replacement of javascripts with more widely accessible code. If accessibility is important to you then this is a must have.  It comes in upgrade packages as well as a stand-alone install. We personally use it on almost every Joomla! website we do (including this one!)