People have been doing this in blender for a long time. You just have to render the object in blender internal with anti-aliasing turned off, then rescale it in the compositor:
Folks usually use blender internal for this, but you can turn off anti-aliasing in cycles by setting the filter width to .01. Setting it to box is supposed to turn off anti-aliasing, but you will get better results by setting the width to .01. The result will still have a little anti-aliasing, but that goes away after it’s rescaled. You will probably also want to limit the max and min bounce to 0 or 1 as well as turn off reflective and refractive caustics.
Here an example blend of how to do this in cycles:toon.blend (547.5 KB)
There is also this old addon for making pixel art in blender:
The output from this doesn’t seem like anything you can’t get from blender internal, and I don’t think the addon has been updated in a long time.