What's the distance from San Francisco to Tokyo? How many calories is 50 g of chicken plus 100 g of potatoes? What was the phase of the moon on October 14, 1066? What about the weather in LA on Jessica Alba's 18th birthday? Stephen Wolfram's computational knowledge engine finally launched late last night, and it can answer all of those questions—and many more.
Wolfram|Alpha is accessible free of charge at wolframalpha.com, and it can compute answers in a wealth of fields, from mathematics, chemistry, and physics to medicine, sports, and music. All you need to do is enter a question or equation in the search field and hit enter. The service is still a little slow at the moment, and getting a question to go through may involve some rephrasing or coaxing. Still, keep in mind the service actually computes answers on the fly, and it generates diagrams in many cases, too.
For an introduction to Wolfram|Alpha, you can check out Stephen Wolfram's demo video and a visual gallery of examples. There's also a variety of toolbars and gadgets, and for developers, Wolfram Research even offers an application programming interface that can tap into the service.