WordPress.com introduces "Gravatar Hovercards" that by moving your mouse over their Gravatar picture in comments, and after a moment the Hovercard will appear showing a person: name, bio, pictures, and their contact info at other services they use on the web like Twitter, Facebook or LinkedIn.
The information shown is taken directly from the person's Gravatar profile. The more info in the profile, the more we show in the Hovercard. This can help you when moderating comments, as Hovercards will appear not only on blogs, but inside your blog dashboard as well.
You can turn Hovercards off for your blog by going to Settings, Discussions, Avatars.