Calais is a free webservice that allows developers to add meta-information to their content (e.g. articles, blog posts). Behind the scenes, Calais essentially adds keyword summaries to content, making it easier for computers to "understand" what that content is about.
Calais does a fairly decent job of tagging content.
It picks out many of the objects within content (people, companies, locations), but it seems to fail to capture the gist of the text (terrorism, earthquake, election). The service will hopefully get much better as the database grows larger. Since it's relatively new, I'm sure they're constantly tweaking their algorithms to improve relevance. Feel free to try it for yourself!