XFN

XFN™ (XHTML Ffriends Network) is a simple way to represent human relationships using hyperlinks. In recent years, blogs and blogrolls have become the fastest growing area of the Web. XFN enables web authors to indicate their relationship(s) to the people in their blogrolls simply by adding a 'rel' attribute to their tags, e.g.:

Basically it's away for you to tell others that you ether know or have met the person you are linking two, sort of like a second way to give them prop's or if you like to be more honest about your links.

If you constantly link to Joe Bloggs some may think it's because you like his style where as infact it's because you know him and have met him and like what he has to say.

Link's

XFN Home Page
XFN Spider RUBHUB
XFN Friendly Link List With Movable Type

Also in the same ilk is FOAF (friend of a friend)

FOAF tutorial

Personal I think FOAF gives to much away but you can chose how much you put in it so I guess it's not all bad.