Content Manager is a free suite of Joomla extensions that allows you to access control who can edit articles from the Joomla frontend.  Version 1.0.4 Beta 1 is now available for download.  Combined several extensions from jxtended.com, Content Manager allows you to create new user groups and limit users to create or modify articles on a per category basis.  For more information see the Content Manager page on The Art of Joomla web site.