Xlisp-stat
xlisp-stat is an open-source statistical scientific package based on XLISP language.
As from xlispstat startup:
XLISP-PLUS version 3.04 Portions Copyright (c) 1988, by David Betz. Modified by Thomas Almy and others. XLISP-STAT Release 3.52.20 (Beta). Copyright (c) 1989-1999, by Luke Tierney.
Many free statistical software like ARC (nonlinear curve fitting problems) are based on this package. It includes a variety of statistical functions and methods, including routines for nonlinear curve fit.