I came across coComment the other day, and it looks quite unique and valuable.
It allows to you keep track of all your comment conversations on the web without resorting to clogging up your RSS reader or your email inbox with comment subscriptions.
I’ve only just started using it, but looks brilliant. With their firefox plugin I can, unobtrusively, see whenever someone has commented on the same posts as me.
All your conversations are aggregated into a single space, but not only that, coComment has also add some web 2.0 goodness, by allowing you to tag your comments, adding community links other people commenting on the same blogs as you, etc. Basically your aggregated source of comment conversations become like a blog all on it’s own.