SculptGL - Update
Follow-up on a previous post about SculptGL - an online in-browser 3D modelling tool. Extra features have been added, such as WACOM tablet support and surface painting, as well as exporting your work to 3D model sites such as Verold and SketchFab.
Also worth sharing is this video demonstration from Gregory Knight creating a human head from scratch - very impressive:
You can use SculptGL in your browser here