Pencil2D is an animation/drawing software for Mac OS X, Windows, and Linux. It lets you create traditional hand-drawn animation (cartoon) using both bitmap and vector graphics. Pencil is free and open source.
Wanna try the bleeding edge version of Pencil2D? Download the nightly builds for 2015 / 2016!
Note: Nightly builds are works in progress so they can be unstable and some functions might be disabled for testing purposes.