Simple Relationship is something along the lines of the rdf module for Drupal, but simpler. It came up quite nicely, there's some bugs (of course) which I still need to fix but overall I'm pretty happy with it.
Unlike the rdf module this one allows you to link to taxonomy terms or vocabularies as well. Maybe not that useful, but it's good to have the option to do it if you want that. Also, users can also create relationships which describe themselves.