BlogMatcher helps people find weblogs that match their interests and find like-minded blogs. When given an URL to a weblog (called "Reference Blog") the system finds other blogs that appear to discuss similar topics.
The basic premise of BlogMatcher is that two blogs that link to the same sites share some sort of topical commonality. If you link to an article in your blog, then the chances are, you'll be interested in reading other people's opinions about the same article.
Blog Matcher and Wiki
Currently the blog matcher will downcase all page names, which makes it kind of unusable for wikis with traditional page names... Are we violating RequestForComments or is blog matcher broken?
Group Think
As SebPaquet wrote http://radio.weblogs.com/0110772/2003/05/14.html#a963, however, it isn't clear wether these kinds of services don't actually promote GroupThink -- you find more people interested in just the same things as you are. This might be great if you need to find new friends and are interested in more information about similar things -- but it might also be dangerous because you will be missing the occasional reminder to put your feet back on the ground.
See also WebLogExamples