Johns Hope's FPlot is a freeware Windows program for plotting and fitting of measurement data and mathematical functions. Why yet another function plotting program? The great advantage of FPlot is flexibility and speed. Functions can be entered as C# source code, which is similar to C, and are compiled on the fly with the C#-compiler included in .NET. This way you can express any function, no matter how complicated, and your function evaluates at the full speed of any compiled .NET program, there is no slow expression interpreter in between. Also FPlot has built in support to numerically integrate and differentiate your functions. If you want to fit your model to measurement data, you don't have to mess around with many predefined models, simply express your fitting model with C# and you are ready to fit the model to your data. So FPlot is a simple but powerful app to do curve fitting. You can also easily refer to third party libraries like dnAnalytics (freeware), NMath from CenterSpace Softare (commercial) or IMSL from Visual Numerics (commercial). As an alternative you can use functions from the book Numerical Recipes in C. Version 2.15 has many UI improvements.