Creating a related content block in Drupal 7 using OpenCalais
Open Calais will take your content and through their web service create metadata for it which the opencalais module can use to create an amazingly diverse variety of taxonomy terms that are applied to your nodes. It is a really cool service.

Once your content had the tags added by OpenCalais you could use one of the modules that provide a "Similar Content" block: http://drupal.org/node/323329 . Most of them are based on taxonomy terms. The More Like This module doesn't look like it has a D7 release but looks like it also uses taxonomy terms.

The simplest method for making a "More Like This" block might be to use Search API Solr or ApacheSolr. If you have full access to your server it is not that difficult to install. Solr's version of More Like This uses Solr's search engine to determine similar content on your site. Also a very very cool technology.