This Joomla Authentication plugin does this:
* Use the Joomla user table to authenticate against and then
* check that that user has a current CiviCRM membership record
* Sets the Joomla Access Level depending on the users Membership Status and / or Membership Type
* Allows users to login with either their username or email address
* Redirects to the configured location based on potential results
CiviCRM is a Constituent Relationship Management System for Joomla! Primarly targeted to the non-profit and public sectors, it will allow you to very effectively manage your organization's contacts, and begin to build a better connection between your CMS and your constituents.