Synfig Studio

Synfig Studio is a free and open-source 2D animation software, designed as powerful industrial-strength solution for creating film-quality animation using a vector and bitmap artwork. It eliminates the need to create animation frame-by frame, allowing you to produce 2D animation of a higher quality with fewer people and resources.

Features:

Can manipulate vector and bitmap artwork

Automatic tweening (independent from FPS)

Multitude of layers of various types (geometry primitives, shapes, gradients, fratals)

Full-featured bone system

Filters and effects for compositing (blurs, distortions, color manipulation, masks)

All color operations use High Dynamic-Range Imaging (HDRI)

Pentablet-friendly tools

Sound support

Linking data on file or parameter-level

Automatic animation using mathematical functions