Favatars is a plugin that enables each comment/pingback/trackback to be accompanied by the website favicon (if available). A little less cryptically,
One of the caveats with this script is that it occurs only at the time the comment is posted, this means that comments from the past will not have favatars associated with them. If you wish to "Favatise" your blog, you can go into your Admin pages -> Options -> Favatars and run the script that will process every comment, pingback, and trackback on your blog and try to capture a favatar from it.
Be patient, this process will take a while and will probably stress your server out a good deal. Be careful with it.
Benny Chandra points out that this plugin may have errors when used in conjunction with Subscribe To Comments 1.4.2, however it works with Subscribe To Comments 1.4.4, so get the upgrade and that fixes the issue.