FAT64 is a FAT32 library for use on the 64drive, a development cart for the Nintendo 64. It is used by the 64drive bootloader and menu.
It can be used for homebrew development with libdragon. It plugs directly into libdragon's dragonfs layer, which makes using it trivial.
Since FAT64 is mostly a dev tool, no tarball is released. Instead you should fetch the latest source from git:
git clone git://lacklustre.net/fat64
You can also browse the source online.