DataFit is a tool for performing nonlinear regression (curve fitting) and data plotting with up to 20 independent variables. You can define your own regression models, or choose from more than 350 predefined 2D/3D models. Results include 2D and 3D plots and a variety of fitting parameters. You can also export code in BASIC or C to use a model in a user-written program. This program includes the addition of 3D color-contour plotting capability and new plot text formatting features, such as superscripting and subscripting. It also provides the ability to create a spreadsheet of evaluated regression models through user-specified intervals, providing interpolated and extrapolated values of the equation. New features in version 9.0 include Intuitive Graphical Interface, non-graphical execution, online help and "tool-tip" text .
Literally about one thousand of my students over the last nine years have enjoyed this wonderful product at Florida Tech within our College of Engineering. I learned how to use this within one minute. Feel free to contact firstname.lastname@example.org for a link to an article on curve fitting. The output is far more complete than the Add Trendline function in Excel, and is easier to use than Polymath (developed by CAChE Corporation - a team of chemical engineering professors). DataFit is also the cheapest true 3-D plotting program. By contrast, Excel plots use the row numbers as category axes when it plots 3-D plots.
The only limitations are an inability to save, print, or copy after the trial period is up.
Oakdale Engineering is very reasonable with regard to site licensing as well.