From TrueCrypt Foundation
Free open-source disk encryption software for Windows Vista/XP, Mac OS X, and Linux
* 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.