GnuPlot - Gnuplot is a portable command-line driven interactive data and function plotting utility.
Qalculate! - Qalculate! is a multiplatform multi-purpose desktop calculator.
Dr. Geo - Dr. Geo, a software to design & manipulate interactive geometric sketches.
NumWorks - The graphing calculator that makes learning math easier.
Giac/Xcas - Free computer algebra system
SpeQ Mathematics - SpeQ is a small, extensive mathematics program with a simple, intuitive interface.