Waver - Description: Google Wave is a great way to communicate and collaborate. Waver lets you:
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.
Rizzoma - Email and IM are linear contextless communication tools.
Roundcube - Web-based IMAP email client
Walkaround - Walkaround is a variant of Wave, based on the Apache Wave code base, that runs on App Engine.
Microsoft Exchange Server - Microsoft Exchange Server is a messaging and collaborative software product developed by Microsoft.