I started using coComment a few months ago, but then dropped it when I couldn’t find any added value.
Now they updated some features that made it worthwhile trying it again:
– there’s a Firefox extension that makes commenting a lot easier
– you can put some code into your blogs comment page to support this
– there’s an RSS feed for all your tracked conversations
Some of those may already have been there previously, but it works a lot better for me now. Give it a try!