OpenGroupware

You wanted an open source alternative to MS Exchange, here you are with OpenGroupware.org:

Gary Frederick, Leader of the OpenOffice.org Groupware Project: "Just to be perfectly clear, this is an MS Exchange take-out."

The FAQ sings a slightly (but more interesting) song: "OGo is something between a mixture of Exchange and SharePoint portal server. It focuses on groupware and collaboration instead of messaging..."