This kind of rendering is called NON PHOTO REALISTIC RENDERING or NPR. I found this blog of Jon Farrel very interesting:
He wrote: "Recently I have been curious about nonphotorealistic rendering. So over the past little while I have been doing tests and checking out the options in achieving nice looking renderings. I was already familiar with ways of creating nonphotorealistic renderings with Renderman, from reading the book "Advanced Renderman Creating CGI for Motion Pictures". I was curious though about what other options existed to create these types of renders. Below are some of my findings.
Interestingly enough, Maya actually has several very good examples of nonphotoreaslistic render styles. My two favorites are below, rendered out in their default settings unless noted.
The other program I found that has very promising results is an opensource line renderer called Freestyle. It allows programmable python shading for the lines you create from you 3d object. Very cool stuff. If you want to check it out there is a link to the left in my links section. The bottom two renders below were done with a style module called JapaneseBigBrush which can be found on the freestyle site.
Overall I am very impressed at the results of my little study. I hope to harness some of these cool shaders and effects for use in a short animation."
A watercolor setup with defualt Maya parameters. Note the very cool bleeding effects present in the render. I composited the render over a paper texture to add to the effect. Other than that there is no post-render work.
this is a frame from an animated sequence I tested. I modified the defualt Maya visor setup to produce these images. The lines you see are a painteffects brush applied to the model which has a transparent shader attached. The Paper texture I grabbed off the internet and used as a camera plane so as to get rid of the need for any post compositing.
Two teapots rendered with Freestyle using the Japanese Big Brush style module.
I am very impressed with the beautiful renders this program puts out. This image is actually two separate renders of the same teapot. I composited the images and took down the opacity of the smaller teapot.