TrueCrypt

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

Features:

Creates a virtual encrypted disk within a file and mounts it as a real disk.
Encrypts an entire hard disk partition or a 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:
- Hidden volume (steganography – more information may be found here).
- 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).
Conceived in 2003.