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.

New in TrueCrypt 7.1

Improvements and bug fixes:

Minor improvements and bug fixes (Windows, Mac OS X, and Linux)
Download for Windows 7/Vista/Xp/2000
Download for Mac OS X