FITYK is a general-purpose curve-fitting program with a nice wxWidget GUI.

It can be used to fit polynomials as well as arbitrary functions to y=f(x) data, and has some particular strength in its ability to fitting to 'peaks' in data, eg in the area of spectroscopy.

It comes with scripting ability using its own simple language, as well as the ability to be scripted using Python.

Fityk is now in Fedora 6-7, so packages disabled for those repos.