so but not sure that all are free :)
Contextfree http://www.contextfreeart.org/
Structure Synth http://structuresynth.sourceforge.net/
Blender http://www.blender.org/
Houdini http://www.sidefx.com/
http://www.paracloud.com/
http://www.bentley.com/en-US/Products/GenerativeComponents/
Groboto http://www.groboto.com/v3/