Synfig is a free 2D vector animation software package. Synfig is capable off producing full feature film type animations.

The usual tools you would expect to find in a vector package are included to help the drawing process. These include cirlces, lines, rectangles and polygons, multiple layers and various filters such as motion blur and bevels.

Because Synfig is a vector package, images and animations will scale smoothly regardless of the resolution.

Synfig is available on various platforms, Windows, Linux and OS X. But bear in mind it is still in development but it's certainly worth looking at. The most unstable platform appears to be Windows at this stage.