Google has released a limited access API for an OpenID, to let Google Account users to login to website using the OpenID protocol. The initial version of the API will use the OpenID 2.0 protocol to enable websites to validate the identity of a Google Account user, including the optional ability to request the user's e-mail address. To request access to the limited trial, visit Google Federated Login discussion group and register using the online registration form.

More info: Google Code