WIMS - A free collection of online mathemathical calculators.
Perhaps some of you already know about this website, but many doesn't I bet.
WIMS @ Université de Nice Sophia Antipolis.
Among (alot of!) other things, it allows you to find f'()/f''()/f'''()/f''''(), integrals (indefinite and definite), factorize expressions, linear solving, matrix multiplication, and the list just goes on! :)
It can also do slightly more advanced stuff; such as "the goldbach conjecture", Fourier / Laplace transforms, sigma and a whole bunch of other things.
It has more than 41 different calculators.
I tend to use it to verify stuff that I do by hand, which would either take to long for my graphical calculator or quite simply take too long to type in on my Casio.