I released the tool I use to edit the 2D animations of my game, Newton Adventure.

nanimstudio is a 2d animation editor with the following features:

• visual editing and preview.
• load/save many animations in one file using a protobuf based format (nanim).
• import/export gif.
• import/export png spritesheets.
• export animated png.
• cross platform (Linux, Mac, Windows).
• free and opensource (BSD license).

Java is needed to launch it.