A rel="" attribute is desirable for links, for example to
* limit the interest of comment spam with rel="nofollow" for anonymous wiki contributions (see [Google](http://googleblog.blogspot.com/2005/01/preventing-comment-spam.html))
* identify page tags with rel="tag" (see [microformats](http://microformats.org/wiki/rel-tag))
* define a social network with rel="friend co-worker met ..." for contacts (see [XFN](http://www.gmpg.org/xfn/))