[JayXon]

If you are interested in this sort of thing, make sure to checkout Funky File Formats by Ange Albertini https://events.ccc.de/congress/2014/Fahrplan/events/5930.htm...

[theoh]

Something else interesting from a few years ago: a compression algorithm that's bijective. http://www3.sympatico.ca/mt0000/bicom/bicom.html

It's similar in the sense that there's more than one possible operation on any file (decompress it, or compress it), and different in that the file type doesn't change.