"SculptGL is an (open source) digital sculpting web app." -- stephaneginier.com
The team at BuildBee has had great fun with this free and simple 3D sculpting tool. Opening it up gives you a ball of clay to work with. Your mouse acts to shape the ball of clay into an organic model.
You can switch the tool on the right hand side to go from painting with a brush, to making creases, to smoothing out your model.
Once you have your 3D model in Blender ready to print you need to export it as an STL:
Note that without a flat base, your print may be difficult to stand up. Additionally, when printing - your design may require supports if the angles are too sharp. This article does a great job explaining print overhangs and how to adjust your design to avoid it.
To add a flat base, you might consider importing your STL to Tinkercad and add any finishing touches.