I am in love. With coComment (thank you, Hugh!).
I've only used it twice (tried to use it on AdRants, but coComment doesn't like it for some reason), but it is very cool and surprisingly easy to use. Essentially, it lets you track, in a central place, the comments that you've made on other blogs and the other comments that were made by others on those posts as well. You can even get feeds of your comments, so you don't have to keep going back to the site .. which is a kind of good thing, because I do find the site a little confusing. You can also put a widget on your blog that shows your comments (I've got it running down in the bottom right column).
Stowe Boyd has a clear breakdown of the features. Jeremy Pepper notes that it gets around moderated comments (on your end, not the blog you're posting on); Ben Metcalfe calls this "semantically forking the conversation". They both raise issues of coComment possibly opening up the floodgates to comment spam.
Right now its beta "invite-only", but I went to their site, filled in a request for info, and received a beta code that day.