Free open-source disk encryption software for Windows Vista/XP, Mac OS X, and Linux

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.
Parallelization and pipelining allow data to be read and written as fast as if the drive was not encrypted.
Provides plausible deniability, in case an adversary forces you to reveal the password:Hidden volume (steganography) and hidden operating system.
Encryption algorithms: AES-256, Serpent, and Twofish. Mode of operation: XTS.
Further information regarding features of the software may be found in the documentation.

http://www.truecrypt.org/