TrueCrypt

Product: TrueCrypt
Website: www.truecrypt.org
Type: Open Source
Requirements: Windows XP/Vista/7

TrueCrypt can create a virtual encrypted disk within a file and mount it as a real disk, or an entire hard disk partition or a device, such as USB memory stick, floppy disk, etc.
This software provides two levels of security: a hidden volume (more information may be found here), and a volume cannot be identified (no part of the volume can be distinguished from random data).

It utilizes the following encryption algorithms: AES (256-bit key), Blowfish (448-bit key), CAST5 (128-bit key), Serpent (256-bit key), Triple DES, and Twofish (256-bit key), and also supports cascading (e.g., AES-Serpent-Twofish).