Onion-skinning is an animation term that refers to making a drawing, then adding the next frame with the previous frame for you to trace from.
It's a litlle like making a flipbook.
Maybe you would like to create a physical flipbook. Watch the video below to get an idea of how onion-skinning works.
A collection of sprite sheets that you could use to help you with your animation.
Have a quick play with FlipAnim to get the idea of "onion-skinning".
If you want to do something more ambitious, download Pencil 2D and start animating. Mobile users can try Flip-a-Clip.
Flare, Synfig and DragonBones are professional animation tools and although Krita is an open source graphics editor a lot like Photoshop, it also has the ability to do onion-skinning animations.
Nice flipbook animation tool, but NO EXPORT option. Also requires Flash.
Lovely little flipbook animation maker (slight shame about the gridlines. See also Pencil, the desktop app)
Professional animation tool
Synfig is a professional animation tool.
Lovely free animation tool for Android or iOS.
Wonderful image editor
Open-source animation tool for Mac and Windows
Lovely desktop animation tool with onion skinning.