Here's an oldie but a goodie. Back in the days when hard drives were small (20 meg was about the standard), a third party app called DoubleSpace was sold to do on-the-fly compression/decompression. It also managed space with much less waste than FAT. Typically, if you had a 20 meg drive, after installing DoubleSpace the size showed as roughly 40 meg. Double space did this by allocating one file with a name like dblspc.bin which took up the entire drive.
I recall a question from a reader in Byte magazine. The question went something like "I have a 20 meg hard drive and after installing DoubleSpace it says I now have 40 meg. There is a file on my drive named dblspc.bin which says it is 20 meg in size. If I delete this file will I get another 20 meg of free space?"