Plaxo have recently released an open api www.plaxo.com/api
I haven't had a chance to look through how powerful it is or easy to use, but Plaxo offers address book, calendar, and task synchronizaton to outlook. I was thinking of using Zimbra open version as my mail server and using Plaxo to sync my address and calendar between outlook clients. The only downside is that there is no easy way to keep the zimbra web client synched. However, a plaxo-zimbra web client connector could do the trick. What do people think, is it within reason?