Disney's Oscar-nominated animated short "Paperman" overlays 2D drawings on CG shapes, giving the computer-generated animation a distinctly (and unusually) old-fashioned feeling without making it feel cheap or low-tech. The artists used a proprietary Walt Disney Animation Studios program called Meander and a technique known as final line advection. Here's a paper on the process [PDF] that was presented at SIGGRAPH 2012.