Tagyu suggests tags for your content. Give it a URL or some text, and it will give you some suggestions for tags.
How does it work? Tagyu compares your text to things that other people have written and how they’ve tagged it. It uses that information to pick some tags that seem appropriate.
I found the idea extremely cool because it fits perfectly on my idea that the push for a common ontology should be collectively constructed and that the real intelligence comes from people and not from the machine.