WordPress.org

This plugin enables "XAuth" support for WordPress (http://xauth.org).

From the website:

XAuth is an open platform for extending authenticated user services across the web.

Participating services generate a browser token for each of their users. Publishers can then recognize when site visitors are logged in to those online services and present them with meaningful, relevant options.

Users can choose to authenticate directly from the publisher site and use the service to share, interact with friends, or participate in the site's community. The XAuth Token can be anything, so services have the flexibility to define whatever level of access they choose.