I have found that while student graphing calculators have become powerful tools not only for advanced math, but for determining derivatives (and integrals!), having those tools available where the results can be put up on a larger screen, or where multiple lines can be displayed in color. I recently came across http://www.numberempire.com/linktous.php where there is a collection of such calculators. While neither WyzAnt or I can guarantee all the calculators will (or will continue to) work as the site describes, I have recently been using the definite integral and graphing calculator, which produced nice, clear & colorful results, which helped clarify a problem we were having with an otherwise very functional black-and-white graphing calculator.
A word to the wise, the online calculators typically require very specific methods of entering functions. For example these calculators often require extra parentheses () or symbols. For example, if I wrote 5 cos 4x, you would assume that the 4 multiples the x, the cosine of which is then calculated and then multiplied by 5. But for these calculators, this order of operations requires all of the relationships spelled out. So it would be 5*cos(4*x) instead.