Freestyle is a free, open source renderer for non-photorealistic line drawing from 3D scenes.
The program uses a shader script model inspired by RenderMan, allowing different shading styles to be written as a script that's interpreted at the render time. The different rendering styles are based on "style modules" that are written in Python programming language.
The stand-alone program, which uses OpenGL and the Qt widget toolkit, can render images based on .3ds files. Work is underway to integrate Freestyle to Blender.