Filehamster monitors changes to any kind of files, and keeps versions. I have seen that many academics just use a numbering system in the filename; filehamster is a bit more elegant.
If you are a programmer, you may know about versioning systems. They are convenient for large projects , particularly for those with more than one programer. However, they are not as easy to use as to justify the overhead when doing simple manuscripts.
Keeping versions helps the flow of writing, since no matter how much you mangle your manuscript, you can always go back to a previous version. And of course you can add comments to versions. Filehamster stays in the system tray and is pretty unobtrusive.
Filehamster is updated often, and each version fixes most of the annoyances of the previous one while adding features.