Synfig Studio is an open-source 2D animation app that can be used to produce both vector-based cut-out animation and film-quality animation using bitmap artwork.
Blender is an all-in-one free and open-source 3D modeling program that allows you to model, sculpt, rig, animate, run physics simulations, render, composite, and even draw and paint all in one application.
FireAlpaca is a free digital painting software designed for both Windows and Mac users, featuring regularly updated brushes, comic panels, animation tools, mesh transform, 3D perspective tool, and more.