From their site: The Google Contacts Data API allows client applications to view and update Contacts content in the form of Google Data API feeds. Your client application can request a list of a user's contacts, edit or delete content in an existing contact, and query the content in an existing contact.
Our API directory now includes 41 contacts APIs. The newest is the AtMail API. The most popular, in terms of mashups, is the Google Contacts API. We list 10 Google Contacts mashups. Below you'll find some more stats from the directory, including the entire list of contacts APIs.