The software packages Octave and scilab can also do this. An example for $y^3 + y^3 - 6x^2y=0$ would be
xmin=-5.0; xstep=0.1; xmax=5.0; // replace with your values
ymin=.... ; // replace with your values
// generate x,y lattice points for plot
[x y]=meshgrid(xmin:xstep:xmax, ymin:ystep:ymax);
// define z=f(x,y), replace with your desired function
z=y.^3 + x.^3 - 6 * x.^2 .* y;
// now perform contour plot of function
contour(z);
and Octave and Scilab are free software and open-source software which can be freely downloaded and used on multiple operating system platforms.