Powerset is one of the first attempts at creating a semantic web search (i.e. using natural English). Watch the video demo embedded below to see how this works. At the moment it only works as an interface for Wikipedia, but for many this is what makes it really interesting. One of the faults of Wikipedia is the interface, layout, and search function, Powerset improves all of this. 

Visit http://www.powerset.com/ to try it out yourself.