Comparing groupware

I now have gotten some time available to play with technology again, and I have set out to find the groupware to use in Arktekk.
Since none of us are using windows operating system, exchange is not really an option, so I have created myself a little short list of requirements that I think a good candidate should have (in no particular order).

• Shared calendars
• Shared imap folders
• Mailing lists
• Good user management preferably LDAP (so that I can also use it with e.g. Subversion)
• Nice looking web-mail, with good ajaxy touch to it
• iCal/iSync support (we are all using Mac’s in our company)
• Able to be deployed on a Linux platform
• Open Source
• As cheap as possible

Here are the ones I am currently looking into :

• Open Xchange
• Scalix
• Kerio
• Zimbra
• eGroupWare
• OpenGroupware

I will write a small post on each and one of them with the pro’s and con’s, and then at the end say which one of them I found to be best. If there are some other candidates out there I have not mentioned, please drop me a comment so I can test it out