Disqus is an external blog comment tie-in. Its kind of like reCaptcha or Akismet, but instead of battling spam, disqus is focused on making it easy to identify yourself to a variety of blogs on various sites by only logging into one. It also provides centralized e-mail subscriptions to follow-up comments - that’s very cool!
Furthermore, it sort of brings some of the cool ideas of OpenID to fruition, and I was glad to see they have an OpenID login. It didn’t work for me, but I’ll try at some point again in the future.
UPDATE: I tried to install the MovableType Disqus plugin, but I couldn’t get it to work. Doh!
Hmm, I thought I just installed disqus on this blog, but it doesn't seem to be showing up....