Free App to Create 3D Sculptures - 123D Sculpt+

Autodesk's 123D Sculpt+ is absolutely the top App for creating 3D models on your Android tablet or iPad. Starting from stick figures, you can sculpt the details, paint the textures and pose your models. You can export your creations to OBJ files for further editing and can share them to the community easily.

This free App is available at Google Play and the App Store. What's more, according to the 123D FAQ, your creation can even be used for commercial purposes freely if you share it to the 123D Gallery!

Get the App at: http://www.123dapp.com/sculptplus

Update on 2015/7/3: The windows version of sculptplus has finally been released! See the official blog for details: http://blog.123dapp.com/2015/06/123d-captures-windows-phones-and-tablets

Update on 2017/5/16: Sad news, 123dapp was closed! See announcements here:
http://web.archive.org/web/20170406144218/http://blog.123dapp.com/2016/12/important-news-about-the-next-chapter
http://web.archive.org/web/20161223192148/https://support.tinkercad.com/hc/en-us/articles/115000144307-Can-I-still-use-the-apps- 
The new successor by Autodesk is web based app https://www.tinkercad.com/, but it seems that it has no sculpting functionality.
The 123D Sculpt (with limited functions) can still be downloaded at:
https://www.microsoft.com/en-us/store/p/123d-sculpt/9nblggh39h05 (Windows)
https://apkpure.com/123d-sculpt/com.autodesk.Sculpt (Android)