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.
Google is making it even easier for developers wishing to implement OpenID with the OAuth. Google has announced that developers can now utilize a "Hybrid Protocol" that combines the OpenID federated login with the OAuth authorization process. The new OpenID OAuth extension makes it easier for developers to implement OAuth through initial authentication using OpenID. According to Yariv Adan on the Google Data APIs Blog: