Fityk is a program for nonlinear fitting of analytical functions (especially peak-shaped) and data analysis software (usually experimental data). There are also people using it to remove the baseline from data, or to display data only. It supports data visualization, separation of overlapping peaks, and least squares fitting using standard Levenberg-Marquardt algorithm, a genetic algorithm, or Nelder-Mead simplex method. It knows common bell-shaped functions (Gaussian, Lorentzian, Pearson 7, Voigt, Pseudo-Voigt) and polynomials, but more sophisticated formulae can be added if needed. It also enables background substracting, data calibration and task automation with a simple script language. It is being developed to analyze powder diffraction patterns, but can be used to fit analytical functions to any kind of data, e.g. it's reportedly used in crystallography, chromatography, photoluminescence and photoelectron spectroscopy, infrared and Raman spectroscopy, and more. Fityk offers intuitive graphical interface (and also command line interface), variouse optimization methods (standard Marquardt least-square algorithm, Genetic Algorithms, Nelder-Mead simplex), equality constraints, modelling error of x coordinate of points (eg. zero-shift of instrument), handling series of datasets, automation of common tasks with scripts, and more. For more information click here. Terms of license are GNU GPL.