The idea of link relationships really gets my gears turning. Wordpress introduced the idea to me with their links section, which includes the XFN link relationships attribute form. In doing a little research, I learned that there are some even more common link relationships, like "prev" and "next", which I should be using already. I'm surprised that search engines aren't championing this more, I would imagine that it would improve the quality of their search results. Perhaps it is more specifically suited for blogs, which often have so many more links? Google did promote the nofollow link awhile back. It was supposed to discourage blog and comment spam, but I haven't heard much about it or it effectiveness lately.
This post about link relationships at Lachy's blog surprised me that it was made in 2004, very cool!