Today Techcrunch posted (here and here) launch of two startups aiming to make blog commenting better. These are Disqus and Intensedebate, both of these do the same or almost the similar thing. They add a comment thread at the end of a blog post just like a blog comment form but with options like rating, threaded commenting etc.
Intesne debate is another commenting system with OpenId support (What is OpenID), comment migration and full feeds of your comments. OpenID support is a better way of doing this stuff, though you can post anonymously with out logging in. One can migrate all the existing comments to intense debate and back to their blog. You can get full feeds of your comments. Provides a recent comments widget to install in your blog. You can style the comments to your blog skin by using external stylesheet.
These services looks interesting and nice addons for blogs receiving tons of comments, that said i won’t be using any of these at the moment.