OpenToonz is a great feature rich and free option, although it may be overkill for someone just starting out. It has a setup similar to the industry standard ToonBoom Harmony.
Pencil2D is a much simpler free animation program. Best for short animations and students. Might be too simplistic for more experienced animators.
Blender is an amazing free program that handles both 3D and 2D, but again the feature density and learning curve to get started can be a turn-off for anyone just getting started.
If you’re drawing on paper rather than digitally, MonkeyJam and a webcam is a great pencil testing setup. Although it seems development for the program has ceased, so compatibility may be limited.
Hopefully this is helpful to those looking for free program options for windows.