The Third Party Accounts Login plugin allows you to enable your users to comment via well known OpenID service providers like Google, Yahoo, Flickr etc. The functionality is similar to that provided by StackOverflow and RPXNow.
The plugin allows the user to provide his/her details via all the famous OpenID service providers (Google, Yahoo etc.) while commenting. A working example of the plugin can be found on http://anantgarg.com
To make this plugin work make sure you have OpenID for WordPress Plugin installed.