I've just been given 20 invites to beta-test True Knowledge, a semantic search engine based on a logic knowledge base. I've been fairly impressed by its ability to utilize user-provided knowledge and its overall good usability, although I've also seen potential for great misdeed by malicious users.
It's mostly a cross between Google and Wikipedia: users provide "facts" about the world, that are stored in a logic format. The interface allows to ask questions that are translated to formal queries over the knowledge expert system, which uses limited reasoning to try and answer the question. This is all quite classic Artificial Intelligence, but the highlights are because of the easiness of use:
- if the query fails, it reverts to a classic keyword-based search. So the user is almost always given some relevant information.
- the process to add new facts is (almost) newbie friendly, which is something really hard to achieve in the dry world of AI
- there is a quality control/assesments of the facts by the users, so that assertions that don't make sense can be voted on and rejected by other users.