Open source equivalent of Outlook and Exchange Server
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. http://www.mozilla.org/projects/calendar/sunbird/index.html 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? http://icalshare.com/index.php?topic=religion&page=2
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.... :\
Try Zimbra
Linking a Google calendar for each room to be reserved seems to work well with our parish, we also have a parish wide calendar, updates can be restricted so only "authorized" staff can reserve a room. It is linked to the website so anyone can access it thru the website (but not update it)
Sounds like a good plan to me! I'm a big fan of Google calendars... partly because, if you ever want to upgrade to an in-house solution, exporting/importing the feeds is fairly easy (especially compared to converting from Exchange calendaring to anything else).

Man...I really didn't dig being an Exchange Admin. You want to deal with people having tizzy fits? Take the email server down for 5 minutes.
Exchange comes with a TON of baggage, but it has matured over the years into a pretty robust, scalable solution with some pretty neat features. Since switching to Mac though...I've never missed it.
I did a bit of looking into this awhile back and the one that stands out, and I came pretty close to implementing once was Evolution:
http://www.novell.com/products/desktop/features/evolution.html
RE: Joomla extensions. JCalPro (commercial) is OK and I supposed it could bent into being a resource scheduler.
Another option is to use a Google calendar for bookings and there's a decent Google Calendar module/component out there: http://extensions.joomla.org/extensions/calendars-a-events/calendars/1625. I used this an event calendar for a Parish website.
Good luck!