You must have Khi3 because it is both useful and fun. It is an all-in-one software. It has more than sixty educational and recreational functions. You can learn mathematics, physics and chemistry by using games. Here are the available features: Games on gravitation, Fourier series, factorizations, triangle interactive geometry, solitary, geocentric and heliocentric planetary co-ordinates, physical and chemical constants, periodic table of elements , Taylor developments, gamma, Bessel, Bernoulli, Fisher, prime numbers, fractionals, equations of degree 3, famous curves and surfaces, game of Carrom (Indian billiards), Divx Bitrate Calculator, Game of Life, Invaders, Tetris, Biorhythms, Simulation of 3D Solar System,names, postal codes, calendar conversions (Gregorian, Julian, Republican) and many others. In the professional version, it includes more advanced features such as calculations of beams, momentum, heat, dynamic, static, 12x12 matrices: inversion, determinants, multiplication, generation of normal distributions, exponential, Cauchy, etc. The GUI is very well-designed and complete. In brief, it is a must to learn while having fun.