Synfig Studio 
Synfig Studio is yet another open-source animation software for Windows, Linux and Mac OS X. It comes in two different variants or binary builds for all supported operating systems and you can choose what suits you the best. The first one is the stable version which is well-tested and proven and sports tons of features. The second is the development version which allows experimental users to try the latest feature as a beta tester and bug fixes as well. 
   
Why you must get it: 
It is an easy, simple-to-use software that will be used by novices and professionals alike. The two different variants also allows users to choose a software that suits their needs specifically.