Here's a great iPhone app that I stumbled upon called DSolver. DSolver is an application that allows you to enter an equation and then perform differentiation on the expression - either 1st or 2nd derivative. Not as full-featured as desktop software, such as Maple, Macsyma, or Mathemtica but pretty impressive nonetheless. Now I just need an integration app!
Here's the equation entry screen, allowing you to input virtually any equation. Here's an equation I entered in the editor - just a random equation here, nothing significant or earth-shattering.
Clicking done takes you to this screen. You can pick partial or mixed derivative, derivative variable, 1st or 2nd derivative, and whether or not to display the solution details.
Here's the result, showing the original equation and the resulting derivative - 1st in this case.
Here are the details of the solution.
For extra credit, here's the 2nd derivative: