WolframMathworld
WolframMathtworld is an online encyclopedia containing some 13.000 entries as well as a number of interactive features. Covers subjects such as algebra, applied mathematics, calculus and analysis, discrete mathematics, foundations of mathematics, geometry, history and terminology, number theory, probability and statistics, recreational mathematics, and topology. Edited by Eric W. Weisstein with assistance from thousands of contributors around the world, the site is updated daily.