The Grease Pencil provides 2d animation tools within Blender's free and open-source full-3d pipeline.

Pencil2D is a free and open-source animation tool, which makes it ideal for all types of users, especially creatives who want to create basic animations without the overhead of software costs.

About a year ago we reported on Pawbyte's Game Pencil Engine, development software that also operates as a 2D editor, that was released for PC development.