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

T r u e C r y p t
Free open-source disk encryption software for Windows 7/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.

Parallelization and pipelining allow data to be read and written as fast as if the drive was not encrypted.

Encryption can be hardware-accelerated on modern processors.

Provides plausible deniability, in case an adversary forces you to reveal the password:

Hidden volume (steganography) and hidden operating system.

http://www.truecrypt.org/