A message from the developer:
I have become too blind to maintain the zunzun.com web site. The open source fitting code is available at:
That page has a link to download the source code as a ZIP file. It comes with many examples, including parallel processing for maximum performance on multi-core computers.
If you would like to make your own web-based curve fitter with functionality similar to that of zunzun.com, my simple tutorial is here: