Well, let's add another one to the mix... Scalix (Score 1)

I might be a bit biased, as I run product management here, but you should also have a look at Scalix (www.scalix.com) - there is a free version available, commercial options as you like it, richest non-Microsoft Outlook connector on the market, AJAX-based Webclient, and CalDAV support is in 11.3, coming in 2 weeks. Won't say much more, please try yourself and tell me what you think! :-) Florian.