Calendaring options

[jim]

Good morning one and all,

I have the opportunity to prevent a client from going to the dark side (MS).  
However, I need a good calendaring solution to offer them.  They have just 
decided to use Outlook but they don't realize they need an Exchange server to 
do that.  I'm tempted to recommend Bynari's plug-in to enable outlook 
calendaring on an IMAP server but I was wondering if there were any open 
source solutions out there that were any good.  Anyone have a recommendation?

Thanks,
Jim