I found pro Fit when it was referenced in a paper that I recently read. I am very happy with it.
pro Fit is unique in the way it allows to very quickly look at data or functions, and very quickly obtain results or high quality graphical output. I see people getting data and then passing it through python code to get a matplotlib plot that is not good for publication, or similar things with Mathematica or MatLab, where getting that final graph is so difficult.
With pro Fit, on the other hand, one can load a data set and boom: see the data in a very flexible preview; Or one can quickly write two lines of python code and boom: see the function in the same preview. Then give the function some parameters and drag the function curve around with your mouse to see how the parameter affects the function and identify optimum starting values for a fit by directly comparing function and data. Or just draw a selection rectangle to select the data points you want to use for a curve fit: no need to filter or rearrange data.
Similarly when creating graphs for publication: just double click and dive deep into a graph to customize every aspect. In addition to that, the graphs are actually part of a general drawing editor, and it is very easy to add LaTeX formulas, photos, pictures, or any drawing that is necessary to clarify the meaning of my data or my plot.
In summary, with its support for python and LaTeX, plus a user interface that allows to immediately interact with data and functions, pro Fit is one of the little jewels for any one who needs to work with data or math. I see from the other comments that it has been around for a long time: no wonder!