Zimbra: open source messaging and collaboration

Zimbra offers Open Source email server software and shared calendar for Linux and the Mac. They do Outlook too, but the Linux/Mac support is the big deal. From Macintouch:

Zimbra Collaboration Suite 4.5 is a server-based, open-source, email/collaboration suite that provides email, calendars, RSS/Atom feeds, tags, and more. It includes support for iSync, Apple Mail, iCal, Thunderbird, Sunbird, Eudora, Outlook, and others. This release adds aggregation of external POP email accounts, email identities, personal distribution lists, the ability to require more secure passwords, email reading in basic HTML mode, email push support in Zimbra Mobile, and more. Zimbra Open Source Edition is available free for Mac OS X and Linux. An enhanced Network Edition is priced starting at $25 per mailbox per year.

The minimal pricing is $350 or so per year for non-profits. That's an excellent deal for a small group. Zimbra may make sense for a small business or startup esp. with a mixture of Linux, Mac and XP desktops.