Along with BackType, I’ve been using CoComment for awhile to try to keep track of all the comments I leave on blogs and websites.
Unlike BackType, CoComment relies on a browser plugin to actively track every comment you post. Once you create an account at the CoComment website, you install their plugin and after that every comment you post gets sent to CoComment as well, associated with your account and with information about where it was originally posted.
It is a wonderful idea that is less useful in its actual implementation. First, it tends to be extremely unreliable — many comments I post never show up at CoComment despite using the plugin. Second, sometimes it is slow to the point where it interferes with posting a comment.
That said, as with BackType it is still better than nothing. Using CoComment, BackType, and a few other tools it is possible to start getting a nice record of all the comments you’re leaving at other websites. It’s still a whole lot harder to do than it should be, however.