Scalix another great alternative to Horde.
Main focus is to be used as a alternative to Exchange server, but provides a nice interface for users.
Also provides a great way to house data locally if they user/client doesn't want to use Cloud Computer
Looking to figure out a way to integrate Zimbra with ClearOS.
Zimbra provides a nice user interface, plus 10 professional users from with their Community Edition.
Great alternative from Horde if the client/user wants to house their data.