Graphite is a free, lightweight open source 2D graphics editor and animation tool that features a procedural, node-based compositor, in addition to traditional layer-based workflows.
GDevelop is a lightweight open source game engine featuring a visual based event system that allows you to express complex game logic without having to write code.
Synfig Studio is an open source 2D animation app that can be used to produce both vector-based cut-out animation and film-quality animation using bitmap artwork.