Groupware is still the big unsolved small-business problem in an open source stack. There have been many, many attempts to fix individual pieces of the problem, and a handful of overarching ones, but I'm not sure anyone has nailed it.

Client side: Evolution kinda/sorta works, most of the time. (And it doesn't run well on Windows or OSX.) The old Mozilla suite has just exploded -- it now takes three (haphazardly maintained -- versions last a few months at most) applications to manage what it used to do.

Server side: open source mail and mailbox servers are among the best, but is there any consensus on the best calendaring ? What clients can even talk to a CALdav server? Outlook can't.