One of the very rare online 3D sculpting and modeling tool, free, without any mandatory registration. Rare!
In a Zbrush style, it permits to model your organic shape and export in various 3D formats (OBJ, PLY STL, plus their proprietary SculptGL format).
The full source is available on GitHub, so as a free standalone version. See upper on its site to find the links; congrats to the author!
Free, No registration, No Download/Install required