Heres a tool for Blog Discovery: http://www.blogstreet.com/blogprofile.html

It has the follwing features:
A blog neighborhood is a list of similar blogs to the one that you entered. You can explore the neighborhood by clicking on the links to the blog, which loads the blog’s page in your web browser. If you like it, there are also RSS buttons available to quickly add the blog to your favorite blog reader/aggregator.

There’s also a "visual neighborhood" tool that uses the TouchGraph visual browser.

There’s also a form to discover "Googlatives," or blogs that Google thinks are similar to the one you’re interested in.