Disqus is a hosted comments solution, a popular one at that. What it does is that it replaces the comment functionality on your blog (or site for that matter) with a hosted one, which means that people can sign in with one username, and you get a lot of cool features without having to mess with plugins or code on your own.
The Mini-Profiles is pretty cool, but the killer feature is the Twitter sign in, available thanks to the new OAuth support (for you techies). Not only can you sign in with Twitter, you can also synch your commenting with your tweets, which is cool. Check out the Disqus blog post for more, or try it out yourself on any of these blogs.