For all who replied, thanks.  It's apparent I need to supply more details.

This customer currently runs Win98 on 10 desktops and one NT 4.0 file server.  
I think I've got them convinced to switch to a Linux/Samba file server and 
add a Linux/Postfix email server.  They want to start sharing calendars 
throughout the office and they just decided to use Outlook 2000 for that, 
though I don't think they realize they need an Exchange server to do that.  I 
would like to steer them away from Outlook, but that might be difficult since 
they use that for their email.

So given that, what I'm looking for is a calendaring package that will run on 
Windows and let them share calendars.  I will try to get them to consider web 
based options as well, since installing a web server is no big deal.

Thanks,
Jim