"If you want to encrypt your sensitive files so that no one can
access them without your personal password or decryption key, you
have several options. But if you want a free, cross-platform, open
source encryption application, try TrueCrypt.
"TrueCrypt encrypts and decrypts files on the fly, as they are
loaded or saved, without user intervention. TrueCrypt can encrypt
files and store them in a container file that acts as a virtual
disk, or make an entire partitions or disk into an encrypted volume
in both Microsoft and Linux. The software depends on a personal
password or key that you create when configuring a TrueCrypt
volume. Without this key, no data on an encrypted volume can be
read (not even offline)..."