Math

Calculator.com

http://www.calculator.com

Use the standard calculator or calculate loans, temperature, currency, time and more.

http://www.unc.edu/~rowlett/units/

Convert inches to meters, pounds to grams or find out about hat sizes, the Apgar Test and almost anything else you can measure.

GoMath

http://gomath.com/

This site "is a free on-line mathematics tutor designed to assist students from K-12, and their parents. ... All GoMath tutors have extensive teaching experience and hold degree(s) in science or engineering from top Universities." Questions may be e-mailed to the tutors (answered within 24 hours) or posted to a message board. Includes a searchable question and answer archive, and more.

Measurements Converter

http://www.convert-me.com/en/

Here you can easily perform online conversions (e.g. metric conversions) for many measurement systems both commonly used like metric and U.S. Avoirdupois and quite exotic like Ancient Greek and Roman.

QuickMath

http://quickmath.com/

"QuickMath is an automated service for answering common math problems over the internet. ...When you submit an algebra or calculus question to QuickMath, it is quickly answered and sent back. Problems may include factoring, graphing, quadratics, inequalities, basic calculus, matrices, and more.