Epic Pen allows you to draw directly onto your screen, making it perfect for leaving drawovers, notes and visual feedback on animations
Epic Pen
Quickposes
Quickposes is a tool for art students, illustrators or anyone who wants to focus on improving their drawing skills. By practicing gesture drawing you will not only get better at recognizing certain aspects of poses, but you will also build a visual library of characters and models.
Line of Action
Line of Action is a really useful site for learning about the human form, anatomy, speed drawing and more. Useful for both 3D and traditional animators!
Animator's Resource Kit 