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!
Privacy: we have no feedback of potential problem with this tool and author. But like many online services, you are here providing them with your own data. Depending of the criticity of your data, this is your responsibility to assess author's reputation and potential misusage, and to check and accept their terms and conditions, or not.
Free, No Download/Install required, No registration
EN 
FR 
DE 
ES 
IT