This is an animation tool I wrote for Maya, which provides a different approach to ghosting/onion-skinning in 3D.
I was watching a recording of a live lecture with Jason Ryan (Dreamworks/iAnimate) when a student asked him if he used ghosting when animating in 3D. He mentioned a couple of reasons why he usually doesn't, even though he uses the Lightbox feature all the time when animating in 2D. I realized I feel the same way about any 3D ghosting solutions I've tried. One problem is that they usually slow the scene down, and another is that they are not that easy to see in the viewport, usually being a faded version of the mesh or a wireframe of it that doesn't read very clearly.
Then the penny dropped for me - 2D animators don't see the fully shaded character in the Lightbox when animating, they see only the lines they've drawn, so the spacing is a lot easier to see. So I set about writing a tool that will give an outline of the character rather than a fully shaded one when ghosted. It's also a little different in that the animator just ghosts specific frames rather than having something heavy that's constantly updating in the scene; for my workflow, at least, this is preferable.