Wolfram Alpha
wolframalpha.com
A computational knowledge engine that answers factual and mathematical queries by computing answers from curated data rather than searching the web.
Why it is useful
Uniquely useful for quantitative questions where you need a computed answer, not a search result — unit conversions, statistical distributions, chemical properties, and mathematical derivations are all answered with source data and methodology shown. For scientists, engineers, and students, it is faster and more reliable than searching for formulas and plugging them into a calculator separately.