I remember there was a freeware file system for AmigaDOS that could store 920 KB on a floppy without using more tracks.
The regular AmigaDOS file system stored 880 KB, Macintosh 800 KB and MS-DOS 720 KB.
I remember there was one utility for MSDOS as well. The name escapes me :(
Those sorts of utils kinda worked on usually if you were on your own computer. But if you really pushed them and the tried to go over to a friends house with one good luck...