They DO exist, here is some info:

for windows there is:
Secure FileSystem (SFS) for DOS/Windows
availble at: http://www.cs.auckland.ac.nz/~pgut001/sfs/index.html

for linux there are the following:

CFS
CFS is a way of encrypting entire directory trees and allowing users to store encrypted files on them. It uses an NFS server running on the local machine and is quite slow due to the nature of the program and algorithms used. More information is available at :
ftp://ftp.zedz.net/pub/crypto/disk/cfs/
or
ftp://ftp.research.att.com/dist/mab/


TCFS
TCFS is an improvement on CFS, but unlike CFS requires kernel patches. It is also only available for the outdated 2.0.x kernels. More information is available in the TCFS FAQ
More info at
http://tcfs.dia.unisa.it/tcfs-faq.html

The Loopback Encrypted Filesystem HOWTO describes another loopback filesystem kernel patch. Many types of encryption can be used, including DES, twofish, blowfish, IDEA, and others.
http://www.kerneli.org/loopback-encr...ilesystem.html

Hope This Helps

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