There is also OpenGroupware.org (the product) available from, you guessed it, http://opengroupware.org It has a long track record commercially and was released as an open source product last year. It may meet you needs as it one of the few that comes with a commercial connector for Outlook clients that is a true mapi connector (hence it cannot be released under a gpl type license) It has a very active community behind it, and excellent commercial or list suport.

Just out of curiosity are you currently using this in a production environment? If so then what impression did you get regarding its stability and interoperability with MS Outlook?