From their site: This API will allow 3rd parties to programmatically synchronize information, address book, calendar, with the Plaxo network. It is also possible for a 3rd party to develop their own Plaxo client which integrates with a contact management application that Plaxo does not support natively. The API currently supports Plaxo-enabling an address book. Calendar, tasks, and notes are not supported yet, but may be added in the future. The API is currently available as a Representational State Transfer (REST) Web Service.