PixiEditor is a free, open source 2D graphics editor that you can use to create video game sprites, animations, pixel art, painting, and even vector art.
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.