[ale] Mail/Calendering alternatives

[David Corbin]

What OpenSource systems can ALE users recommend as an Exchange 
replacement?  Specifically, Calendaring (mail is kind of no brainer, and 
those are the two main areas of interest).  The replacement must work 
well with both Linux and Windows clients, and not require a browser, 
though browser access is a good thing.  It also must be mature enough 
that it is easy to install and maintain.

Thanks
David