I’ve now had the opportunity to install and attempt to integrate two open source PHP-based groupware systems (PHProjekt and phpGroupWare), and the verdict is not good. Although they mostly work as advertised within a limited operating environment, both systems have serious problems with stability, reliability, support, and documentation.
It’s looking more likely that we will go with a best-of-breed approach. In this scenario, we would select separate packages from separate vendors and glue them together with some common interface. Microsoft provides products for such a scenario, allowing one to tie together Office, Outlook, SharePoint, Content Management Server, SQLServer, and Server together with Exchange. Unfortunately, licensing fees, disdain for MS products, incompatibility with everything else, and security concerns have kept us from selecting the Microsoft route — except as a last resort.
However, a couple open-source alternatives to MS Exchange exist:
Both currently use WebDAV and XML as an interface for communication between components of a groupware system.