Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Postfix - Postfix is a mail transfer agent (MTA) that routes and delivers electronic mail.
Event Hunt - Startups and Tech Events Curated By Locals
iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Zimbra - Zimbra is trusted by over 500 million users to increase productivity with a complete set of collaboration tools while maintaining total control over security and privacy.