Available:

• http://www.opengroupware.org/en/index.html
• http://www.novell.com/products/openexchange/
• http://hula-project.org/Hula_Server
• http://www.novell.com/products/netmail/

And yes, thats 3x novell!

Proprietary:

• Groupwise (Novell again)
• Domino (Lotus/IBM)
• Exchange (Micrsoft)

Comments:

• http://www.jwz.org/doc/groupware.html