Autograph 30 day trial: http://www.autograph-maths.com/download/index.shtml
GraphCalc (old but will
do the trick): http://www.graphcalc.com/download.shtml
LabPlot (another option
to try out): http://labplot.sourceforge.net/download/
QtiPlot (looks easy to
use): http://soft.proindependent.com/qtiplot.html
For more, check http://en.wikipedia.org/wiki/Category:Free_plotting_software