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.
Waver - Description: Google Wave is a great way to communicate and collaborate. Waver lets you:
Kolab Community - Kolab Groupware is a secure, scalable, reliable, flexible, full-featured collaboration suite.
Rizzoma - Email and IM are linear contextless communication tools.
iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).
BSI Knowledge Manager - Central document management & knowledge base