After the news about the .Net Facebook SDK, LinkedIn has released an API to integrate LinkedIn in your own applications. LinkedIn uses OAuth to authenticate.
“The LinkedIn API utilizes OAuth as its authentication method. OAuth is a standard model for negotiating developer authorization and granting access on behalf of specific users for performing API requests. One of OAuth's strengths are the many third party and open source libraries that exist, allowing developers to authenticate with LinkedIn quickly and in a similar manner to how they may authenticate with services like Twitter, Google, Yahoo, and Netflix.”
More info: http://developer.linkedin.com/