Truecrypt

Free open-source disk encryption software for Windows Vista/XP, Mac OS X, and Linux

Main Features:

• Creates a virtual encrypted disk within a file and mounts it as a real disk.
• Encrypts an entire partition or storage device such as USB flash drive or hard drive.
• Encrypts a partition or drive where Windows is installed (pre-boot authentication).
• 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, Serpent, and Twofish. Mode of operation: XTS.

Link: http://www.truecrypt.org/