I installed Zimbra as my mail server a couple of days ago.
Zimbra is still in beta but I am impressed. They also provide 'Outlook Connector' in their 'paid-for-support' version for corporate environments. The amazing thing is they just popped out on the scene some 6 months ago.
At the heart of Zimbra are all popular open source tools like postfix mail server, mysql for database, clamav for antivirus, spamassassin for anti-spam (even Intel was using SpamAssassin for last 5 years, they recently move to IronPort), apache+tomcat for web UI with AJAX, Lucene for message indexing and Java for all the backend work.
They also provide a very nice web based GUI for Administration plus all the goodness of command line utilities for batch processing. I was able to move all my mail from qmail to Zimbra without any trouble. They also support clustering, and virtual domains, so this server is indeed suitable for large installations. Each and every message that enters the system is indexed, so searching for messages is extremely fast. Also unlike Exchange, if you send the same message to many uses on the same system there is only a single copy of that mail maintained. I am hosting this server on a 4 year old Pentium 4 2.0 GHz machine with 512MB RAM and it seems to be doing just fine for my domain (which is about 8 users). They claim that on a 3GHz, 2GB machine this server can handle 10K mails per 30 minutes, which seems good for corportions too.
The only thing Zimbra doesn't support yet is Mailing Lists. I am planning to add that support myself. This should be straight forward enough since the MTA is postfix. This is the reason I shut down chai_time mailing list. It should be back up in a couple of weeks.