Kolab is a free and open source groupware suite. It consists of the Kolab server and a wide variety of Kolab clients, including KDE PIM-Suite Kontact, Horde Webfrontend, Mozilla Thunderbird and Mozilla Lightning with SyncKolab extension and Microsoft Outlook with proprietary Kolab-Connector PlugIns.

Read more about Kolab:  Basic Concepts, Main Features, Kolab 2.x Clients, Kolab 3.x Clients (in Development), History