Disqus = Blog Comment Zen
If you run a blog or comment regularly on a blog and haven't installed Disqus then do it now. Well if you comment on a blog regularly, then let the blog owner know they need to get with the times.
Basically it's a centralised system for blog comments. If all blogs ran this system then you could have a centralised database of all your blog comments (across multiple blogs) and be able to track the replies. Who's ever commented on a blog post then forgotten about it. While the replies to your comment stack up.
I also love the Gravatar support which centralises the visual representative of you.
Installation of Disqus took 1 minute for me on Moveable Type. You can then customize the look and feel of the comments through the Disqus admin page.
The only annoying part is Disqus cannot go through previous comments and add them automatically. So Disqus will only be used for new posts onwards. Which is a shame.
Update: I just found the Widget section of Disqus. Check below for an update on the hot topics on the blog. Fantastic. How did I ever blog without this tool