http://www.linuxlinks.com/Kernel/File_Systems/
There is FUSE
http://lwn.net/2001/1115/a/fuse.php3
http://www.inf.bme.hu/~mszeredi/avfs/
http://sourceforge.net/projects/avf
http://ftp.sourceforge.net/avf/fuse-0.95.tar.gz
There is LUFS
http://lufs.sourceforge.net/
CODA works too