Synfig

Synfig is a 2D open-source animation software. It is capable to produce vector artwork and also can work with bitmap images. The main concept of Synfig is "tweening" - you can define object positions or shapes of vector objects at certain points of time and program will interpolate in-between frames automatically. You can also use bones to control your animation on higher level. With Synfig you can easily create motion graphics and cut-out animations for product explanation videos, tutorial videos, and more.