bhGhost is a very useful onion skinning tool for animation in Maya. By now it's been around for almost four years with many new features and updates being released over time,
so creator Brian Horgan decided to put together a video to give you an overview of how the tool works. It's a good place to start if you're new to bhGhost, and even if you're already familiar with it the video contains a lot of useful tips of things that you might have missed.
bhGhost is free to download. To get it, visit this page.