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

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.

via truecrypt.org