TrueCrypt is Free open-source disk encryption software for Windows XP/2000/2003 and Linux.

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).

    * Encryption algorithms: AES-256, Blowfish (448-bit key), CAST5, Serpent, Triple DES, and Twofish.
      Mode of operation: LRW  (CBC supported as legacy).

Useful Links 

TrueCrypt Home Page

TrueCrypt Documentation

TrueCrypt Download Page