Pencil 2D is open source and it has existed for quite a while but development stopped. Luckily a new effort to revive Pencil 2D was made and many updated versions has emerged since. Pencil 2D has a very nice feature that makes zooming in/out or scrolling left/right quite easy. Have a look at this piece of software if you are thinking about animating. Here’s a few videos from a couple of test projects I made with Pencil 2D.
More samples from me:
Links: Pencil 2D webpage