OpenCalais

Thomson Reuter provides OpenCalais as a Web service to index texts (http://www.opencalais.com) into several categories. And there exist a module for Drupal integrating the OpenCalais functionality into Drupal Websites (https://www.drupal.org/project/opencalais). If this module is enabled we have possibility to set certain characteristics in /admin/config/content/opencalais.
First we have to register at Thomson Reuter to get an access key that has to inserted in the settings dialog.
Then we can goto certain content nodes to decide whether they shall be analyzed by the indexing service. Different categories can be used. Social tags seems to be the most promising category. I have enabled the service for challenges and solutions.