The software packages  [Octave](http://www.gnu.org/software/octave/) and [scilab](http://www.scilab.org/) can also do this.  An example for $y^3 + x^3 - 6x^2y=0$ 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.
 
 - http://en.wikipedia.org/wiki/Scilab --  http://www.scilab.org/

 - http://en.wikipedia.org/wiki/GNU_Octave --   http://www.gnu.org/software/octave/