Jpixel - JPixel is an editor for making pixel art animations and tilesets.
Sprite Fusion - Design your game with Sprite Fusion, the ultimate free online tilemap editor. Design 2D games with ease, direclty export to Unity Tilemap and Godot Scenes. Ideal for retro games!
Synfig Studio - Just another WordPress site
Aseprite - Aseprite is an art program dedicated to the creation of pixel art.
DragonBones - DragonBones is an open source project for 2D skeletal animation.
LibreSprite - Free and open source program to create animated sprites.