Has anyone had any luck with any of these Exchange Server replacements that run
on Linux? Does anyone know of any other decent replacements for Exchange Server
that does calendaring, address book sharing, instant messaging & collaboration,
and other Outlook 2000's features?

OpenMail 7
http://www.openmail.com/cyc/om/00/showfile.cgi?100-1623

MailOne
http://www.openone.com/software/MailOne/index.htm

Bynari Insight Server:
http://www.bynari.net/Products/products.html