The benefits to Open Source software are pretty clear. With the recent release of Toonz as open source, has perhaps seen its first proliferation —The OpenToonz freehand stroke.
Blender Developer Campbell Barton decided to tinker with some of the open source code from OpenToonz. In doing so, he has ported over OpenToonz freehand drawing tool into Blender, as an experimental add-on.
Simply download the repository as a zip file, and install it from within Blender. You can add a curve in edit mode, and use Shift Left Mouse Button to draw strokes in 3D space. There are a few options that you can adjust in the options tab for each stroke. Campbell Barton notes that the Freehand Stroke tool handles are calculated in screenspace, but the function can operate on 3D splines without a hitch.
There are some noted differences to the OpenToonz version of the freehand stroke tool. Support n-dimensions (can be used for calculating a 3D curve & pressure… or other custom-data), and replace 2D bound-box handle clamping with vector length clamping.
There is a strong possibility that freehand stroke may be eventually added into Blender.
— Chris Offner (@ChrisOffner3D) April 5, 2016