Check out the Calais Drupal module, based on the Calais service from Thomson-Reuters.
The Calais modules for Drupal (found at http://drupal.org/project/opencalais) are feature-rich, highly intuitive and designed to make it easy for Drupal users to automatically metatag their content, generating rich Semantic metadata that can be shared via a simple key and integrated into the larger content universe.
The screencast shows what an implementation would look like. I'm ready to use Drupal's RSS aggregator or FeedAPI module to pull in some content and see how Calais auto-tagging would work.
Of course, now I want to see what Calais does to library items imported into Drupal, see how Drupal would expose RDF and other stuff!