Typekey is a system where a person has one login, and with that can leave comments upon many movabletype sites. Indeed, if Firefox is used, the password might be remembered by the browser!
It's a bit of a pain to log in, I know - but it's a one time thing for many sites. Typekey is pretty new, but I'd expect to see its use increase over time - comment spam on some sites has become a big problem.
I do plan to enable unregistered comments (which will be held for moderation). Unfortunately I have some problem with the templates at the moment which is prohibiting this - and reusing the default templates doesn't seem to fix things. I've got the appropriate boxes checked in the configuration, so I'm a touch confused. I'll have a good delve when I can. In the meantime, typekey looks like a good way to go - and typekey comments appear straight on the site (if you're not held for moderation by MT-Blacklist!)