Actually, we're looking into some collaboration suites at present. I've been playing with Zimbra (CentOS server) but am still feeling my way around it. I'm almost afraid to play with its postfix config file so far. Also, of course, there is the worry about MS buying Yahoo and what might happen.
Interestingly enough, to my surprise, some cost studies, (not by MS) showed that Exchange might actually be cheaper in some cases. The problem with Zimbra (and another one, the name escapes me, Scaler, or something similar) is that they have yearly licensing, so that the cost gradually goes over the one time cost of Exchange. (The comparison was also based on getting RedHat with premium support, which adds something like $1300 annually to the equation.)
I'll take a look at Citadel too. I see a mention of it on Zimbra forums. We might wind up with a postfix/dovecot solution as well, at this point, we're looking at all options.