http://www.truecrypt.org/
Main Features:
* Creates a virtual encrypted disk within a file and mounts it as a real disk.
* Encrypts an entire hard disk partition or a storage device such as USB flash drive.
* Encryption is automatic, real-time (on-the-fly) and transparent.
* Provides two levels of plausible deniability, in case an adversary forces you to reveal the password:
1) Hidden volume (steganography – more information may be found here).
2) No TrueCrypt volume can be identified (volumes cannot be distinguished from random data).
One can not have too much security these days.