Blog Wandering is a free tool that allows a user to explore a blog visually: you choose a blog from which to start and you get a map composed of the author and of her/his commenters, then you can click on a commenter and open the map of her/his blog, if she/he owns one, and so on.
Double clicking on a user opens a blog in a new tab, right clicking opens the gravatar profile.
If you have any suggestions, I'd be happy to hear them!