XFN – XHTML Friends Network is a way to tag relationships. Here is an example:
<a href="http://jeff.example.org" rel="friend met">...
The attribute “rel” uses a set of simple keywords to describe the relationship between the author of the page linking from to the person represented by the page (blog or home page) linking to. Interestingly, XFN was designed to only allow positive or neutral relationships.
What can you do with XFN? Well … in principle it will allow the graphing of relationships between people representative sites. rubhub.com is an XFN lookup engine to which you can add your page and then, once it has crawled your site you can see the relationships linking out or in.
I discovered this playing with Word Press which has incorporated it into its links/blogroll feature.