Socialauth - OAuth library in Java, Authenticate users using their Gmail, Twitter, Facebook login
Socialauth provides support to authenticate users through external oAuth providers like Gmail, Hotmail, Yahoo, Twitter, Facebook, LinkedIn, Foursquare, MySpace, Salesforce, Yammer, Google Plus, Instagram as well as through OpenID providers like myopenid.com. It could be easily integrated, create a page where users can click on buttons for the above providers or other supported providers. Just call SocialAuth and you can get all their profile details.
It supports Importing contacts from Google, Yahoo or Hotmail. Support for importing friends from Facebook, followers from Twitter and contacts from LinkedIn is available, but currently Facebook, Twitter and LinkedIn do not provide email addresses. UPDATE: Hotmail has stopped providing email addresses.