Sure, here are the requirements.
Shared calendaring and appointments.
Ability to view all calendars and appointments (non-private, of course).
POP3 & IMAP e-mail support
Easy to use interface for the basic users
A perk would be the ability to see who's logged in when you are.
A shared message/news board for announcements.
thats pretty much it I think.
I've already seen eGroupWare and moregroupware, but looking for others to try and compare.