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.

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.

Eventbrite - Discover Great Events or Create Your Own & Sell Tickets

Kolab Community - Kolab Groupware is a secure, scalable, reliable, flexible, full-featured collaboration suite.

Bigtent - BigTent is a web-based organization and communication portal for groups.