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) and hidden operating system.

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.

Further information regarding features of the software may be found in the documentation.

Source : http://www.truecrypt.org/