Mathematics

www.mathwords.com, an online math dictionary covering beginning algebra through calculus, written and published by me, Bruce Simmons

Khan Academy, a collection of instructional videos helpful for all kinds of beginning and advanced math (and statistics)

Mathworld, an online math dictionary covering advanced topics (calculus and beyond), created by Eric Weisstein and published by Wolfram (publishers of Mathematica®)

WolframAlpha, an online computational engine from Wolfram Research, the makers of Mathematica®.