Two simple examples


Minixfs (developed by Andrew Tanenbaum for his Minix OS)
It was meant to be simple and small, and Unix-like
14byte filenames
64k inodes
It shows its age for real systems, and it's too big of uC.


RomFS (introduced in Linux-2.2 for embedded systems)
Read-only data structures, all files are concatenated
No support for user and group
No support for date and time
No support for physical links.
Created, as a file, by a user-space tool ("genromfs")
Still maintained and useful in some situations
It is around 5kB of compiled code