TV Paint Alternatives
FREE and open source painting program that includes animation as of version 3.0. Also great for concept art, texture and matte painters, illustrations and comics.
Clip Studio Paint Ex
Free Trial, and then a 1-time purchase fee (includes updates). Has an embedded aniamtion system. Use for illustration and animation options. When you create an animation in Clip Studio Paint, it automatically creates a raster (also commonly known as bitmap) layer. Thus anything you draw on these layers will be rendered in raster format. You can also choose to work in vector line format.
Procreate (drawing pad app only, not desktop)
Has a simple animation set up that works nicely for roughs and simple storyboards. You can work with raster and vector-based lines. Also great for concept art and illustration, as it has masks, layer modes, effects, and is infinitely customizable for brush/pencil tips.