I keep meaning to post about this, so now I am!
When I started really focusing on the blog this month, I also wanted to keep track of comments: both the ones I write elsewhere and the ones people write here. Yes, you can bookmark each blog and keep track that way, but I found that I kept checking pages that had no new comments, which is an enormous waste of time.
I tried several sites that were supposed to help with this, but they were all made of fail. Then I found co.mments.com. THIS has worked for me. It's a simple registration form, and then you can tell the web site (either through a bookmarklet on your browser's toolbar or by going to the co.mments site) to watch a particular blog post.
At first it kept asking me to log in again, but I changed my privacy settings to allow the site to store cookies and since then it's been fabulous. I've had no trouble with it keeping track of blogger, livejournal, or wordpress blogs (as long as each blog is set up to show each post on its own page) and I can see all old and new comments when I visit the site's page. (You can also see it as an RSS feed, but I find I prefer the site.)
I have absolutely no affiliation with the site: I just thought it might be helpful! :)