T r u e C r y p t is a free open-source disk encryption software for Windows Vista/XP, Mac OS X, and Linux
It’s main Features are:

  • 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.

http://www.truecrypt.org/