SculptGL - Update
Latest version of the in-browser natural sculpting 3D modelling program by Stéphane Ginier has many more useful features.
As well as built-in WACOM tablet support, import/export of popular 3D file formats and more sculpting methods, the most impressive addition is the ability to paint your model freehand, in both metallic and flat colours.
Also, you can send your creations straight to Sketchfab which allows you to embed your 3D creations on a webpage.
There is a lot to play around with, and it is powerful enough to be a basic yet rich 3D modelling tool.
You can try out SculptGL here [with some navigational instructions here]