I inherited a sysadmin position for a mid size educational organization (75 people) and we completely lack any form of collaborative software-- even the email list is in paper/pdf form. I thought some sort groupware would dramatically communication. My situation:

-The needs are very basic; we need calendaring and shared contacts. Stuff like you would see in MS outlook/exchange... notification of new meetings, email reminders of meetings, etc.
-Needs to be web accessible.
-Everyone uses or is migrating to thunderbird, and I'd to leverage that.
-Everyone has an existing email address, the servers of which are managed by the school system, which is out of my control.
-It needs to be free/cheap and run on *nix.
-Easy to administer-- this isn't my primary job so I can't afford to baby sit it.

I looked at zimbra, but I think it conflicts with our external email handling because zimbra seems to want to do everything under the sun.

Any thoughts/experience/flames appreciated.

Probably doesn't completly fit your description but you can have a look at http://chandlerproject.org/.

I've always been impressed with eGroupware
http://www.egroupware.org/

We are currently using eGroupWare and I would recommend it as a good groupware/collaboration system that can be integrated with many existing applications across a wide variety of platforms.

If everyone is using Thunderbird it would be fairly trivial to setup an LDAP server to provide a unified contact list also.