Synfig is a cartoon animation package with tweening- vector based so the results can be output at any size.
From their site:
“Synfig is a powerful, industrial-strength vector-based 2D animation software package, designed from the ground-up for producing feature-film quality animation with fewer people and resources. While there are many other programs currently on the market to aid with the efficient production of 2D animation, we are currently unaware of any other software that can do what our software can.”
Need reviewer for this!