I've just discovered this beta of a new service called 'coComment' thanks to TechCrunch. It looks like it will be a really interesting service.
From their site:
When you comment on a blog, coComment keeps a copy of your message on a special page - called "your conversations" - that allows you to quickly see your comment and where it was posted, with a link to the original site.
You post your comment using a bookmarklet rather than the blog interface, and it is supposed to work with loads of different platforms. You can also share your comments with your own blog's readers, by easily posting them to your blog. In addition, the system will alert you when other people reply to your comment, so you don't have to keep going back to the blog.