Pixelorama is an increasingly advanced free and open source sprite editor, and just recently it had a massive new release with plenty of new goodies.
With Pixelorama 0.7 they added in layer locking, cel linking, a cel-based timeline, you can export animations to gif, a UI overhaul, shortcut bindings for tools, a pixel-perfect mode, layer rotation, a new zoom tool and the lisr goes on. It's quickly becoming quite fully featured! Check out the new release trailer:
There's other tools out there for making pixel art but a little competition doesn't hurt. It's also a reminder of how versatile Godot Engine is, as a free and open source game engine it can also be used for making other tools like this.