Pencil2D is an easy, intuitive tool to make 2D hand-drawn animations. Pencil2D is open source and cross-platform.

"Onion-skin" visual differences between a reference document image and a scanned copy

Bloat-free animation program for Linux using SDL2, GL, Xi, and ImGui

Stop-Motion software for DSLR cameras with onion skin features.

[Development fork] Pencil2D is an animation/drawing software for Mac OS X, Windows, and Linux. It lets you create traditional hand-drawn animation (cartoon) using both bitmap and vector graphics. Pencil2D is free and open source.