Very good Software for nonlinear Fitting is Origin Lab. See link: www.originlab.com You can use also Matlab for nonlinear Peak Fitting, but there it is more like programming: www.mathworks.com
PeakFit (http://www.sigmaplot.co.uk/products/peakfit/peakfit.php) is a specific software to fit multiple peaks . It has a variety of functional forms and its help includes all theoretical information about them.