So, I’ve finally set up Zimbra, after a bit of equivocating on whether to use it in the first place.
It’s really slick; imagine a Free Software replacment for GMail, Google Reader, and Google Gears, along with all the enterprisey management stuff that the likes of Active Directory/Exchange, Groupwise, and Notes have. It takes care of all the thorniness of setting up a the Postfix MTA, and gives you advanced calendaring features (and it can sync up with webcal ics calendars, too!).
Very slick, and it seems to take a best-of-both-worlds approach on several issues: it has a classic Folders-based interface, yet the concept of tagging is pervasive throughout the system; it provides both SOAP and RESTful interfaces to its datastore; it’s (mostly) Free Software while providing a commercial subscription model attractive to the more traditional managers.
 Actually, there is one slight issue of contention; Zimbra’s internals are MPL licensed, but the GUI component is licensed under the so-called “ZPL”, which is actually the MPL with a badgeware clause added. The whole attribution argument is a shitfest I don’t want to get involved with, but I do prefer pure Free Software licenses over badgeware licenses; licenses that place limits on your ability to modify software present a slippery slope, as today it may only be display-logo-please requirements, and tomorrow it could be nasty DRM or patent clauses.