TrueCrypt

TrueCrypt - Free Open-Source On-The-Fly Disk Encryption Software for Windows Vista/XP/2000 and Linux
Free open-source disk encryption software for Windows Vista/XP/2000 and Linux

Main Features:
* Creates a virtual encrypted disk within a file and mounts it as a real disk.
* Encrypts an entire hard disk partition or a storage device such as USB flash drive.
* 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: LRW.
Further information regarding features of the software may be found in the documentation.