Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Microsoft Exchange Server - Microsoft Exchange Server is a messaging and collaborative software product developed by Microsoft.
Facebook Local - Discover places to go and events happening near you
OBM - OBM is a free messaging and collaboration platform useful for just a couple to many thousands of users with strong support for mobile devices.
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.