Among the math apps, I can say MathStudio (very costly, but worth spending if you know how to use it) and MyScriptCalulator (No cost) are the best user friendly math apps available on Android. Though I have also installed Cctave, Maxima, Gnuplot & PariGP spin offs, but they work best as desktop apps.
MathStudio usage is almost like Mathematica (but nothing can beat wolfram Mathematica). It allows you to solve all kinds of mathematical problems from among vector/function plotting, indefinite integrals, differential equations, fourier transforms etc. But the best thing I liked about it is that it builds extremely neat 3D plots.
MyScriptCalculator allows you to enter simple arithmetic calculations in a natural way and then uses handwriting recognition to get you the answer (similar to an ipad app Pankaj showed me). I now do not use calculator until unless I have to do some form of conversion.
One can also use GeometryPad and Dr. Geo for teaching/learning simple co-ordinate geometry. Almost all functions are dynamic and parametrised.
For small simulation purpose, you can use octave,Maxima,Gnuplot & PariGP spin offs by writing small programs (can write huge simulations by attaching an external keyboard to tablet/Notebook). But they need to be configured properly.