Scalix, formerly HP OpenMail.
http://www.scalix.com/
At the time of writing, Feb. 2007, the open source version of Scalix comes with a 25 user license for their "premiere" services, which includes Outlook MAPI support! Not bad at all. Thankfully, now that VMWare images and other virtualization are making Linux servers reasonably easy to deply (and somewhat easier to maintain), maybe I'll finally have a chance to recommend something other than Exchange and Outlook?
http://www.zimbra.com/
The latest AJAX Groupware thing. Outlook connectors require the non-free license.
Insight by Bynari - not really OSS, but cheap
http://www.bynari.net/groupware.html This is relatively cheap GroupWare that runs on Linux. $0 for 10 clients. I have my doubts, however, Exchange and Outlook Web Access keeps getting better. Then again, OWA works best under IE5+ (ActiveX).
Calendar and Contacts sharing (only, I think). Also has CardDav integration extension for Thunderbird.
https://sogo.nu/about.html#/features
Tags: email, server, smtp, outlook, exchange, group ware, calendaring