purpose: program for data processing and nonlinear curve fitting. Has command line and graphical interfaces.
project homepage: http://fityk.nieto.pl
reason: There are no alternatives to this software, and this particular solution is extremely powerful for research and education.
open source: yes
tarball to 1.3.1: https://github.com/wojdyr/fityk/archive/v1.3.1.tar.gz
additional note: This package requires xylib (library) by the same author. Also open source.
tarbal to xylib 1.5: https://github.com/wojdyr/xylib/archive/v1.5.tar.gz