MacTutor History of Mathematics is on online project oirganized and sponsored by the University of St Andrews in Scotland. It contains biographical data, topics in the history of mathematics, and other mathematical topics.
Wolfram|Alpha a search engine that computes answers using Wolfram's breakthrough technology & knowledgebase, and is relied on by millions of students & professionals in disciplines like math, science, nutrition, history, geography, engineering, mathematics, linguistics, sports, finance, music and more.
Online Encyclopedia of Integer Sequences an online compendium of integer sequences like prime numbers, the Mersenne primes and so one, based on the original print monograph of Dr. Sloane.
The pi page Curiosities of and trivia about the number π.