A Digital compositor is a program involved in the assembly of multiple videos, images and color inputs to make a final video composition.
Blender 3D has a mature and stable digital compositor. However, it never hurts to have other free alternatives.
Natron is a new free opensource Digital Compositor that looks quite promising. Like Blender, this compositor is node-based. It has an interface similar to Nuke. Typically, compositors come in two varieties, namely node-based and layer-based.
Natron supports OpenFX and therefore is intended to be able to install OpenFX plug-ins. More information on this compositor is at http://natron.inria.fr/ . It can be download from that website as well.
Below is an animation made with a combination of Blender 3D and Natron.
As of this writing, no manual can be found, which makes learning and using it fairly obscured. However, the interface is quite straightforward.
Several free compositor projects have ended in untimely disappearance. Hopefully, this one does not follow the same fate as the rest.