DragonBones

DragonBones is a free 2D skeletal animation tool for game development, letting artists rig a character once and reuse the skeleton across multiple animations.

Skeletal animation (bone-based, rather than frame-by-frame sprite animation) means smaller file sizes and smoother movement than traditional sprite sheets, and DragonBones exports runtimes for most major game engines (Unity, Cocos2d-x, Egret, Pixi.js, Godot via community plugins).

The editor itself is a free desktop app; the runtime libraries for each engine are also free and open source, making it a common choice for indie and small-studio 2D game projects on a budget.