Open Source Exchange alternatives

My boss, being the... uh, cost-controlling person that he is, asked me if
there's an open source alternative for Exchange.

Specifically, he's looking for something that allows him to share his
Outlook calendar with the corporate secretaries.

Anyone knows what alternatives are available out there?

Needless to say (but I'll say it anyway), my preferences would be:
something stable in portage tree, something unstable (but not masked) in
portage tree, something stable in some overlay tree.

Rgds,