TrueCrypt Data Encryption Application
TrueCrypt is a free and open source encryption software for Linux, OS X, and Windows. There is a simple GUI interface to the application for each of the operating systems to create encrypted containers of data. Once the container has been mounted by supplying the correct password, the data in the container can be used in real-time just like data stored on a regular disk drive. For Windows it offers the option of whole disk encryption which can prevent anything on the drive other than the small boot partition from being accessed unless the boot password is known. This is extremely impressive and powerful software that should be used by anyone who stores any type of sensitive information on their computer.