Disk encryption

Free open-source disk encryption software for Windows Vista/XP, Mac OS X, and Linux
TrueCrypt is a software application used for on-the-fly encryption (OTFE). It can create a "file-hosted container" or write a partition which consists of an encrypted volume with its own file system, contained within a regular file, which can then be mounted as if it were a real disk. TrueCrypt also supports device-hosted volumes, which can be created on either an individual partition or an entire disk.

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.

http://www.truecrypt.org/