I visited a well known architectural practice this week who specialize in freeform organic architecture. They had asked me come in to present Revit Architecture 2009 to see how they may be able to incorporate into their workflow. However, in our discussions about software that they currently use they advised me to take a look at an application called TopMod. I wrote down the details and Googled it when I got home.
This is an amazing application! As the website says "TopMod3d is a free, open source, portable, platform independent topological mesh modeling system that allows users to create high genus 2-manifold (watertight) meshes". I knocked this up in about 2 minutes!
Agreed its not Revit, but I would certainly recommend you take a look if you like creating organic forms within a computer environment. TopMod has limited export capabilities, but I was able to save the file in .obj format and was then able to import this into 3dsmax.
I then exported the form from Max as a DWG and was able it import it into a Revit mass family. This is only a mesh, so you won't be able to cut floor plates from the mesh.
Quick draft render in Revit......