I've just signed up for coComment, and you'll see a new box on the right hand side of my blog titled 'Blogs I am commenting'.
Why is coComments so great and worth signing up for (and it's mac compatible with Safari):
1. You don't have to have a blog: Once signed up an in use, you'll have a web page that shows all the blogs you are commenting on, and your comments, and any replies in comments from other people. In other words your custom page with your conversations.
2. Your blog: If you have a blog you can share your comments and conversations from other blogs on your blog, for people to find other places you are posting.
3. RSS tracking: And if you don't want to track your comments and others on a web page, you can get an RSS feed and read them all in your news reader.
It's in beta stage, and looks well worth taking for a spin. Thanks to Tech Crunch for the heads up