Blender is a powerful and free software for creating 3D and 2D animations. One of its features is the grease pencil tool, which allows you to draw and animate directly in the 3D viewport.

Blender's Grease Pencil is a powerful tool for drawing and animating 2D elements in a 3D space. You can use it to sketch your storyboard frames directly in Blender, or import them from other software.