What is Wolfram Alpha? It is a computational knowledge engine. It answers specific knowledge questions. It is kind of like Google for exact questions. I think it is kind of novel and kind of crazy. Some say the way of the future. It must be since it knows the answers to such deep questions: What's the speed of an unladen swallow? To understand what it is you really have to use it or if you have a hundred minutes of your life to burn you can have Wolfram himself explain it in excruciating detail.
Update: Physics World has an article about Wolfram Alpha here.