There's this very interesting system for classifying the category a piece of text belongs to, called IPTC Media Topics. It helps you determine what subject topic a particular document / text piece belongs to. Here's an example of the hierarchy categorization offered by Media Topics:
Media Topics is a taxonomy of over 1100 terms designed to help categorize text and understand what subject it belongs to. It was originally released in 2010 and usually gets updated once a year.
This can have a lot of interesting applications when combined with NLP, for instance, and I'd love to see more people using it in their work.