coComment is an interesting site. It's purpose is to be a single place where you can keep track of all of the comments that you have posted to various blogs. It displays these comments in "Conversations", where you can see the thread of comments. You can also tag your comments, to help you organize them.
The way to use this is to either install a FireFox Extension, or to install a bookmarklet. Once you have written the text of a comment, you press the coComment button. The comment text, as well as a link to the post and the blog url are sent to coComment.
Alternatively, I have modified my template to automatically submit comments to coComment. Without being logged in, the comments are just put in for the blogs conversation, but you can also log in and have the comments added to your conversation.
coComment also allows you to make your "conversations" private. This would allow you to use this as a service as a way to personally keep track of the places that you post comments.