http://code.google.com/apis/contacts/
The Google Contacts Data API allows client applications to view and update Contacts content in the form of Google Data API feeds.
Import a user’s Google contacts into their web or desktop application
Export their application’s contact list to Google
Write sync applications for mobile devices or popular, desktop-based contact management applications
The API uses the Google Data protocol which is based on AtomPub (the same standard that Microsoft is now adopting). All contacts feeds are private and apps can use either the Google Client Authentication API or the Google Web Auth API for authenticating.
http://blog.programmableweb.com/category/apis/
