Here's what they say about it:

I've started using it in my main animation folder, on this file of my character Pepper that I've been working on, which had 34 copies already with slight changes, and I think I'll be using the annotations so I'll know what's different in each file. You can open any of the revisions in the list nondestructively, and if you want to stop using it entirely, you can just copy the revision archives into your main folders.As an artist, you know how long it takes to create a finished piece of work. And, anyone that’s used a computer understands how painful it is to lose something you’ve been working on for any length of time. Whether your file becomes corrupt, is accidentally deleted, or you inadvertently saved over a good file, you know the pain. There are methods for dealing with these problems that include special naming conventions, archiving backups to removable media, or moving files to a different folder, but these solutions are archaic compared to what FileHamster offers. Even worse, they all disrupt your workflow.
FileHamster presents a much better way.
It is simple to use. Tell FileHamster which directory or file to watch and get back to work. Every time you save, rest easy knowing you’re not losing any of your precious work. In the background, FileHamster automatically detects that your file has been saved and creates a revision of it. In addition, you have the ability to make notes each time you save to help you later identify a specific file, or use those notes to see a detailed account of the work you've done on a particular project. Go to http://www.filehamster.com/ProductTour/ for a demo.
MOGware also provides a powerful plug-in architecture that enhances FileHamster’s functionality. Plug-ins are freely downloadable at http://www.filehamster.com/Downloads/ including a zip plug-in for saving your revisions in a more compact form, a floating notes plug-in to help organize comments, and more.
FileHamster is simple for beginners, but customizable for more advanced user. Put archaic methods where they belong, in the past, and move forward with FileHamster!
Get it FREE at http://www.filehamster.com
EDIT: Ha! I just noticed this -- it also serves as a Recent Documents list for Anime Studio files, if you right-click on the icon in the system tray!
Oh, and yeah, this is a Windows-only program.