http://drupal.org/project/similar
http://drupal.org/project/relatedlinks
http://drupal.org/project/relativity
http://drupal.org/project/user_related_content
http://drupal.org/project/relatedcontent
http://drupal.org/project/clipper
http://drupal.org/project/relationship
http://drupal.org/project/nodereferrer
http://drupal.org/project/cck (nodereference/userreference)
http://drupal.org/project/category (?)
taxonomy?