Developer: Elliot Hughes
Price: Free
Mathdroid turns your phone into a scientific calculator.  It can do all the basic functions any calculator can, as well as some more advanced calculations such as algebra and trigonometry.    The multi-line display allows you to see your last three equations, which is incredibly helpful in coalition with the copy and paste feature.  Mathdroid isn’t going to replace your TI-89 math majors, but for the rest of you it is a free alternative to a very expensive calculator you’re barely going to use anyway.  Download Mathdroid if you need to use a calculator infrequently.