If you don't want to run an MS Exchange server, but you do want to have the groupware functions such as sharing calendars, emails, notes, contacts, etc. there are a few semi open alternatives you can try which also support phone synchronisation.
Most support webbased functionality and allow you to attach any client to it, some come with their own suite of clients.
But if you want to use your Outlook client, all of them require you to buy a connector.
OpenGroupware.org Ogo
Scalix
Kolab
Zimbra
Zafara
Citadel
Open Xchange
Most work quite a bit like Outlook, but Citadel is totally different, out of the box.