EGroupware is a free enterprise ready groupware software for your network. (GPL)

Evolution provides integrated mail, addressbook and calendaring functionality to users of the GNOME desktop. (GPL)

Horde Groupware Webmail Edition is a free, enterprise ready, browser based communication suite. (LGPL)

Kontact is a personal information manager and groupware software suite for KDE. (GPL)

OBM is enterprise-class messaging and collaboration platform for workgroup or enterprises with many thousands users. (GPL)

Open-Xchange is the information and knowledge headquarters of a company. Open-Xchange combines e-mail, appointment, contact and task management with intelligent document management. (GPL)

phpGroupWare is a fully featured, web based messaging, collaboration and enterprise management platform. (GPL)

Zarafa open source offering consist of the Zarafa server, Z-Merge, the Zarafa IMAP/POP3/iCal gateway, the Zarafa Administration Console, the Zarafa WebAccess and many other tools and services. (AGPL)

Zimbra is a client and server platform for next-generation messaging and collaboration. (MPL)