Derivative checker
This applet will try to check whether you've calculated a derivative correctly.
Note: Computers are extremely finicky with functions, so there's a chance that the computer will think your answer is incorrect even when it's correct. In that case, take a look at the purple graph, which is the difference between the actual derivative and your derivative; if the purple graph appears to just be 0, that means your derivative is most likely correct, and the computer is just being difficult!