Easy to use on-the-fly encryption program.
TrueCrypt is a program for establishing and maintaining an
encrypted volume (data storage device). On-the-fly encryption
means that data are automatically encrypted or decrypted right
before they are loaded or saved, without any user intervention.
No data stored on an encrypted volume can be read (decrypted)
without using the correct password/keyfile or correct encryption
keys. The entire file system is encrypted (i.e., file names,
contents of every file, meta data, etc).