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.

Friend of a friend okay, so what is this you say. Well, it is about your place in the Web, and the Web’s place in our world. FOAF is a simple technology that makes it easier to share and use information about people and their activities (eg. photos, calendars, weblogs), to transfer information between Web sites, and to automatically extend, merge and re-use it online.
There are an estimated 93 million OpenID owners out there, a lot of them not knowing where to use their OpenID, how to use it, or even not knowing that they own an OpenID at all.
The Internet has grown to an adolelescent stage. The first decade we witnessed from birth through infancy a powerful rise of the Internet. Some declaring that the Internet was to be a source of freedom, others evangelizing an Internet of shared knowledge and yet others fueled by greed tried harnessing and then proprietorial this knowledge for monetary reasons.