Maya Non-photorealistic Rendering Framework (MNPR) is an Open Source Stylistic Framework for Expressive Rendering.
Plenty of 3rd party renderers such as V-Ray, Arnold, and others have NPR or non-photorealistic rendering methods. Most of those tools tend to focus on toon shading including line-art style rendering for 3D scenes. They are also typically an effect that requires rendering. If you are using Autodesk Maya, there is a new open source framework for real-time artistic non-photorealistic rendering, called MNPR as part of research work by Santiago E. Montesdeoca, Hock Soon Seah, Amir Semmo, Pierre Bénard, Romain Vergne, Joëlle Thollot and Davide Benvenuti.
The system works directly in Maya’s viewport and offers a method a tool pallet that lets you art-direct the stylization of the scene that has multiple levels of control. The results looks painterly and impressive while giving instant rendering right in the viewport.
You can download MNPR for Mayas version 2017 and 2018 that includes a simple installation routine. Check out Maya Watercolor Stylization based on MNPR here.