Google has released a Contacts API as part of their existing GData feeds framework. The new 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." Contacts are shared among Google apps like Gmail, Google Calendar, Google Reader and more. As one use-case, Google says this API allows web developers to import a user’s Google contacts into their apps, as well as export their app’s contacts into Google.
What's interesting is that the contacts are tied to a Google Account, not necessarily to a Gmail account, so Google could release a separate address book for those who don't use Gmail.
Google, API, Gmail, Google Calendar, Google Reader