Synfig allows you to draw 2D animation and vector drawings without manual “tweening” which means that you have to draw each frame individually. This allows for fluid motion and non-choppy movement for animation.
This allows a lot of indie type development with fewer resources and personnel and with more powerful software at your disposal.
I’ll have to say that it sure seems like a pretty good deal considering it can support Windows, linux, and MacOSX. Unfortunately for MacOSX, currently there is no builder for this or someone to test so it’s been taken offline until they gain someone interested to help with the project. I’m unsure if there is Aqua code or if it supports tablets offhand, but if it does, then it definitely gives the other vector based programs a run for their money.