I was really impressed by Naiah's side bar -- a collection of links to her comments on other blogs. Co-comment (see the very bottom of my page) does that.
When I signed up, I tried it for a couple of weeks without success. Then, I tried it again and it worked. I'm not sure what made the difference, but now it works. e.g. (I know, nothing earth shattering in my comments, but it is neat to see the concept working):