MATHEMATICS

WolframAlpha - Computational knowledge engine. Compute anything.

Ask Dr. Math - Ask Dr. Math is a question and answer service for math students and their teachers brought to you by Drexel University's Math Forum. 

How Many? A Dictionary of Units of Measurement - If you ever needed to know information about hat sizes, wind chill scale, paper sheet sizes in the US vs. the rest of the world, and Apgar scores, this is the place for you. This collection of units of measure incorporates metrics, English units, and international systems.

Mathworld - Mathworld is a web-based mathematics encyclopedia for students, educators, researchers and math enthusiasts.  

Math 10 - A well-respected website covering all areas of mathematics created and maintained by mathematician living in Bulgaria. 

The Khan Academy - The Khan Academy gives students yet another way to access 1800+ video tutorials that teach the ins-and-outs of algebra, geometry, trigonometry, calculus, statistics, finance, physics, economics and more.