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
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 features in 7.1:
Full compatibility with 64-bit and 32-bit Mac OS X 10.7 Lion
Minor improvements and bug fixes (Windows, Mac OS X, and Linux)
Download it at www.truecrypt.org.