Came across this site this morning which provides some usefull mathematical tools. It was useful for me to check a derivative of a function.

Other topics include:

Derivative calculator

Integral calculator

Definite integrator

Limit calculator

Equation solver

Expression simplifier

Factoring calculator

Expression calculator

Inverse function

Taylor series

Matrix calculator

Matrix arithmetic

Prime numbers

Number factorizer

Fibonacci numbers

Bernoulli numbers

Euler numbers

Complex numbers

Factorial calculator

Gamma function

Combinatorial calculator

Fractions calculator

Statistics calculator

LaTeX equation editor

Graphing calculator

(and yes there are a lot of ads too)