Synfig - Free Vector-Based 2D Animation Software

With this program you can produce feature-film quality animation. One of the key functionalities of Synfig is automatic "tweening", meaning that it generates intermediate frames that are required to produce the illusion of smooth, fluid motion, similar to the tweening feature of the Adobe Flash authoring software. This eliminates a significant amount of manual labor. The Synfig package has been released under the GNU General Public License. Source code and binary installation packages for Ubuntu and many other Linux distros can be found here. See here for screenshots.