WP - OpenID
OpenID is an open standard that lets you sign in to other sites on the Web using little more than your blog URL. This means less usernames and passwords to remember and less time spent signing up for new sites. This plugin allows verified OpenIDs to be linked to existing user accounts for use as an alternative means of authentication. Additionally, commenters may use their OpenID to assure their identity as the author of the comment and provide a framework for future OpenID-based services (reputation and trust, for example).
The developer of this plugin Alexandre Passant, PhD student in the Semantic Web and Social Software area has done a terrific job with this plugin. So for you Wordpress users this is a great tool.