Top Mod http://code.google.com/p/topmod/
Chaoscope http://www.chaoscope.org/
Apophysis http://apophysis.org/
K3Dsurf http://k3dsurf.sourceforge.net/index_fr.html
are free and very funny to use for crazzy results!

But maybe I don't know what is "Generative design" ?