If you've never heard of Disqus, you either don't notice the logo or don't comment on very many blogs. It's a centraized service running comments forms. Just install an add-on into Wordpress, which already provides commenting......

Why?! I know, I use reCAPTCHA, but that service provides, by virtue of being centralized, a genuinely better CAPTCHA (well, unless everybody wanted to get into the business of digitizing books, that is...). Yes, Gravatar doesn't really need to be centralized, and I use it, but there isn't a more widely-adopted, decentralized, system out there yet.

I know Disqus allows you to track replies to all your comments, but getting replies on any decent blog (like this one) is simply a matter of checking the "Inform me of replies" box and entering your email. Authentication (if you want it) a box labeled "OpenID" and maybe a chooser for a few preprogrammed services. Getting a list of comments you've made in the past? If you really want it, just record when you comment (it's not exactly a usecase I've heard of outside Disqus ads).

If anybody can name a good reason to use Disqus, as opposed perhaps to Wordpress with a few plug-ins, or if you design your own site, just to swallow the pill and write it, let me know.