Saw the announcement for Hula -- apparently Novell is open-sourcing their email+whatever NetMail software. Guess I'm interested because of the calendar piece, in some combination with email, but less than compelled. There are so many uninspiring calendar applications. Will have to take a look at Hula later to see if they have done better. If done well this could be interesting.
For a colorful but worthwhile assessment read Jamie Zawinski's take on Hula.
We should not forget Chandler which is still floating out there in chunky-vaporware-land. Could end up interesting.
Tim Bray touches on Sun's use of groupware (or what amounts to something of the sort). A few year's back I set up an internal wiki for my work-group, and used and recommended IM as a supplement ... guess we're not too far behind. On the other hand [snipped comments only relevant within the company] there are bits that could be better.