Wikipedia
Romfs
In computing, romfs (ROM filesystem) is an extremely simple file system lacking many features, intended for burning important files onto an EEPROM. It is available on Linux, and possibly other Unix-like systems.
It is very useful as an initial ROM holding kernel modules that can be loaded later as needed. It is very small, code wise.
The description of the filesystem layout is available on LXR -- romfs.txt.
Bo Brantén has created a RomFS file system driver for Windows NT/2k/XP.
Nikolay Aleksandrov has written RomFS image extraction and manipulation tool called romfser. Primarily targeted at the BSD family of operating systems because they lack support for RomFS.