Again I feel maybe this should be in the core as its a common requirement and very much part of the file system mantra.
If not maybe a app, but I really do feel revisions, ocr, preview, nextant and many other apps could benefit from a common method of hidden files for process use.
Revisions no brainer that hidden previous versions need to be stored somewhere, hidden native preview formats for fast viewing, ocr the full text renditions of documents that nextant tries to catalogue.
That just a few apps that to certain extents are all providing there own individual methods and I am more prone to think this should be a core common method where system admins can toggle a view and administer shadow copies and shadow volumes also in a standard common method.