James,
Zunzun is a great tool. I'd be interested to hear your (and anyone else's) view on how it compares to the curve-fitting ability of other commercial and free software such as:
Origin
Wolram Mathematica
SciDavis, QtiPlot, LabPlot
Fityk, MagicPlot
SigmaPlot
E.g:
Are the range of available equation types as good?
Is there key curve fitting features Zunzun has that other software doesn't? (and vica versa). E.g. function finder?
Thanks,