Jack Slocum - WordPress Comments System built with Yahoo! UI:
"New comments can be General Comments as well, or they can be "Block" comments. A "Block" is any paragraph, image, title or code in a post. When you select a block to comment on, it is highlighted in blue.
Comments can be posted anywhere in an article. If you see something and you wish to comment on it, click on the comments bar directly next to it.
To view the comments posted by readers, click on the yellow indicator next to that "Block" on the comments bar."
(Via Ryan Eby.)