EncFS is a program which provides an encrypted virtual filesystem for
Linux using the FUSE kernel module.  FUSE provides a loadable kernel 
module which exports a filesystem interface to user-mode.  EncFS runs 
entirely in user-mode and acts as a transparent encrypted filesystem.

Usage:
mount: encfs /path/to/encrypted_dir /path/to/mountpoint
umount: fusermount -u /path/to/mountpoint