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.