Krita is a professional FREE and open source painting program. It is made by artists that want to see affordable art tools for everyone. Has an intuitive user interface that stays out of your way. The docks and panels can be moved and customized for your specific workflow. Once you have your setup, you can save it as your own workspace. You can also create your own shortcuts for commonly used tools. Over 100 professionally made brushes, stabilizers for them in case your hand isn't the steadiest, built-in vector drawing tools, customizable and constructable brushes, wrap-around mode for seamless textures, and a resource manager to import and export tools and packs from other users.

Supports 2d animation. Multiple layers and audio support, thousands of frames on the video timeline, onion skinning for tweening, drag-and-drop of frames, shortcuts, and performance tweaking.

Source code: https://invent.kde.org/graphics/krita