Similar to the ghosting feature found with Armatures, this script makes several ghosted layers of an animation. Its explained best as 'a method to have some sort of ghost image of an animation at keyframes defined by the user'
There is a video demonstration on Vimeo here, which shows the basic usage of the script (click image below)
(Note that onion skinning has been sped up since this video was made)
Theres 2 scripts to download, and instructions on their usage can be found below.
Script 1 - GUI
Script 2 - Spacehandler
Instructions (by Bart Crouch on BA):
"1 - go to python text-editor
2 - text --> open --> # select the spacehandler script on your harddisk
3 - text --> open --> # select the gui script on your harddisk
4 - go to the 3d view and press CTRL+UParrow (to enlarge the window)
5 - View --> Space Handlers Script --> Draw: onionskin_spacehandle
6 - CTRL+DOWNarrow (to get the window back to its normal size)
7 - go to the text editor and press ALT+P
Steps 4 and 6 are only necessary to be able to see the option in step 3, because it often happens that it falls off screen otherwise."
You can see the small thread on Blenderartists.org, which follows the development of the script, with pictures, video, instructions and the scripts for download. Plus you can leave comments and ask any questions.