Fityk is a program for nonlinear fitting of analytical functions (especially 
peak-shaped) to data (usually experimental data). There are also people using
it only to display data or to remove baseline from data.

It is reported to be used in crystallography, chromatography,
photoluminescence, infrared and Raman spectroscopy and other fields.

Fityk knows about common peak-shaped functions (Gaussian, Lorentzian, Voigt,
Pearson VII, bifurcated Gaussian, EMG, Doniach-Sunjic, etc.) and polynomials.
It also supports user-defined functions.

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.

WWW: http://www.unipress.waw.pl/fityk/