TrueCrypt, Free Open-Source On The Fly Disk Encryption Software

TrueCrypt is the 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.