Wolfram|Alpha

http://www.wolframalpha.com/

From the Wolfram Alpha website:

Wolfram|Alpha's long-term goal is to make all systematic knowledge immediately computable and accessible to everyone. We aim to collect and curate all objective data; implement every known model, method, and algorithm; and make it possible to compute whatever can be computed about anything. Our goal is to build on the achievements of science and other systematizations of knowledge to provide a single source that can be relied on by everyone for definitive answers to factual queries.

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Wolfram|Alpha launched yesterday and it's an useful complement to a general-purpose search engine like Google. Wolfram|Alpha doesn't search the web, it uses data from authoritative databases and an engine powered by Mathematica to obtain the results.
"Wolfram|Alpha aims to bring expert-level knowledge and capabilities to the broadest possible range of people—spanning all professions and education levels. Our goal is to accept completely free-form input, and to serve as a knowledge engine that generates powerful results and presents them with maximum clarity," explains the "about" page.
Even if the homepage's simplicity reminds you of Google, it's a good idea to check the examples page before entering a query. Alpha can't handle any kind of query, so it's important to know its strengths: advanced Math calculations (graphs, equations, prime numbers computations, matrix operations), conversions, facts about weather, population, food, minerals, people, places.
You can enter simple things like numbers, dates, words, HTML color codes, chemical formulas and you'll get a lot of interesting information. The service shows intuitive visualizations and comparisons to better understand the information.

Wolfram|Alpha