Free open-source disk encryption software

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.
* Encryption is automatic, real-time (on-the-fly) and transparent.
* Parallelization and pipelining allow data to be read and written
  as fast as if the drive was not encrypted.
* Provides plausible deniability, in case an adversary forces you
  to reveal the password:
  Hidden volume (steganography) and hidden operating system.

WWW: http://www.truecrypt.org/