Synfig is a free vector-based 2D animation program that you can use to 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.
On the project web site you can find more information about the latest release, get the source code or Linux binaries. The Synfig package has been released under the GNU General Public License. You can also view various examples of animation produced with Synfig, including the Morevna project.