TrueCrypt

TrueCrypt - Free Open-Source On-The-Fly Disk Encryption Software for Windows Vista/XP, Mac OS X and Linux:

T r u e C r y p t
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.