Does anyone know if there is an open source and/or free alternative to Microsoft Outlook and Exchange Server?  I found Mozilla is beta-ing something called Sunbird.  But this doesn't look to be able to book conference rooms or invite others to meetings, just a schedule sharing program.  But it does link to a cool site that appears to have come calendars already created with the Catholic liturgical schedule.  Has anyone used those before?

Alternatively, has anyone seen a module or add-on for Joomla/Drupal that allows for the "rooms to be booked" feature.  My parish has a big problem with people booking the same room at the same time.  I'd like to just add it via the website(Joomla 1.4) but I haven't found anything like that(allowing multiple users to book, but not the same location at the same time).  Basically exactly what Outlook does but not Outlook....  :\