Natron is a powerful Digital Compositor that can handle all of your 2D/2.5D needs.
It is built around industry standard open source tech such as OpenFX, OpenEXR, OpenColorIO and OpenImageIO. The feature set includes realtime playback/RAM cache, multi-threaded rendering, 2d tracking, rotoscoping, grading, animation via f-curves and 3rd party OpenFX plugin support. All contained within a familiar, intuitive interface with an XML based project file format.