Recently I learned of a company called Sugar Lab and it amazingly prints .stl files as sugar art creations. I hope to be exploring their services in a future project! Below is a design test I created of an abstracted cherry blossom flower that could be 3D printed in sugar.
Shapeways Creator has interesting tools and options for 3D printing (to their services). I wish it enabled export of .stl files. Maybe it does and I just have not found the function yet. Here is a design I created with Sugar Lab in mind for output.
Finally, Sculptris is a great tool for organic 3D designing. I am just beginning to explore it. Below is a wasp I am making in Sculptris. It looks like I will need to download Blender to convert the object files of Sculptris to stl files. I would like to create components in Sculptris and import them as stl files into Tinkercad to complete the forms. The very bottom image is a Sculptris design created by my nine year old when I walked away from my computer for five minutes!