Grasshopper
Even though Grasshopper initially can seem intimidating and overly complex it is one of the most powerful and intuitive parametric design tools available. It functions as a plug-in to the modeling tool Rhino3D. The plug-in is free and there are no restrictions. You can save your work in Grasshopper as many times as you like. The Grasshopper website has an incredible engaged, helpful and inspiring community.
Rhino3D
Rhino is powerful three dimensional modeling software. It uses parametric curve and surface reprensentation to create infinitely accurate double curved geometry. It is easy and intuitive to work with. The full software can be downloaded as an evaluation version.
Enter Geometry 