ENVE, or Enve is Not a Video Editor, is a new in development 2D animation software, completely free and open source. Even in this early state, Enve is remarkably capable animation tool, with a full suite of drawing tools for both vector and result art, including MyPaint brush integration.
ENVE is described as:
Enve is a new open-source 2D animation software for Linux and Windows. You can use enve to create vector animations, raster animations, and even use sound and video files. Enve was created with flexibility and expandability in mind.