A big trend in design for 3D printing is the use of "organic" modeling software such as Mudbox, Zbrush and 3D Coat that mimic traditional artist techniques such as sculpting in clay. These tools give artists immediate feedback allowing them to conceptualize ideas quickly without being encumbered by the curves and splines found in more commonly used CAD packages.
Unfortunately, accelerating the creative process comes at a price making many digital sculpting tools out of reach for some budget conscious artists. One cost effective solution is Sculptris from Pixologic (makers of Zbrush). Another interesting option is the web-based digital sculpting application developed by Stephane Ginier called SculptGL.
Stephane's ScupltGL tool allows any starving artist with a web connection to delve into organic design for 3D printing. Awaiting users who visit his site is a large sphere of digital terra-cotta "clay" on the screen. There is no software to download; the site itself is the application and users of Zbrush will instantly be familiar with this floating ball of virtual putty that can be pushed and pulled with the strokes of a mouse.
If you are new to organic modeling, SculptGL is the perfect place to wrap your head around the process. The interface options are kept to a minimum inviting new users to explore without being overwhelmed. Menus can be found on the right and left, giving access to a range of common modeling capabilities such as symmetry, material properties, subdivision levels and pressure sensitivity. Important functions such as undo and import/export can be found here as well. Users can export and import both .obj and .stl files. Also included is the capability to connect to other web-based 3D applications such as Verold and Sketchfab allowing users to share designs to the greater 3D art community.