The OpenCalais module for Drupal integrates features developed by Thomson Reuters. It automatically adds metadata to Drupal nodes, which can then be configured to set the content taxonomy, such as categories or tags. OpenCalais is free, but use of the module requires registration to obtain the Calais API key. The OpenCalais module is compatible with Drupal versions 5, 6 and 7.
Drupal's tagging system is known as taxonomy, with specific terms serving as tags or categories for specific content, called nodes. Semantic tag modules for Drupal use Web services to automatically analyze the content in a node. The modules then affix the taxonomy term to the content, or display the list of related taxonomy terms in a Drupal block.
The Drupal Amplify module uses the OpenAmplify Web service to automatically scan your Drupal nodes for terms to add as taxonomy. The module is self-contained and has no external module dependencies, although an OpenAmplify API key is required. It's available free on the OpenAmplify website. Amplify is only available for Drupal version 6.
The Drupal Inform module does not automatically add taxonomy to nodes. It does, however, display a list of related taxonomy inside a block placed in the sidebar or in the content area beneath the node. The Inform modules uses the Inform Web service, which is utilized by large websites like CNN, CBS, BET and The Village Voice. It's only compatible with Drupal 6.
The Drupal Yahoo Terms module uses the Yahoo Term Extractor Web service to automatically analyze the words within a node. It then attaches related taxonomy tags to the node. The Yahoo Terms module lets you batch analyze and tag a group of nodes. Processing on this scale is useful if you want to automatically tag a large group of posts. Yahoo Terms is compatible with Drupal version 4, 5, 6 and 7.