Designed by Wolfram Research, the creators of the powerful mathematical computation engine Mathematica, Wolfram Alpha is an online computational engine that basically allows you to harness the power of Mathematica commands from the web without knowing any Mathematica. In fact, you only need to use pidgin English. Moreover, Wolfram Alpha often displays a lot of useful informaiton related to your query that you might not otherwise have thought of asking, and also often supplies the corresponding Mathematica functions that allow you to carry a more in-depth exploration using Mathematica. Wolfram Alpha also has a lot of stored data in biology, geography, medicine, chemistry, and other areas that it makes use of in computations.
See also their blog.