Check out Twiggi groupware. I got the latest version running on SME 5.1.2. It is web based and provides many of the Exchange features like a contact groups, shared group calendars, global email address book, to do list. It does not have public folders, but that is what ibays are for. Getting it working was slightly challenging, or you can purchase Twiggi preconfigured from myezserver.com for under 100 bucks.
Good luck,
ryan