The ultimate open-source encryption program, TrueCrypt is loaded with powerful features that those concerned with protecting their data from thievery should want--and have.
It offers 11 algorithms for encrypting your private files in a password-protected volume. You can store your encrypted data in files (containers) or partitions (devices). TrueCrypt works hard to offer powerful data protection, recommending complex passwords, explaining the benefits of hidden volumes, and erasing telltale signs of the encryption process, including mouse movements and keystrokes. Though the interface may not be immediately intuitive, its powerful, on-the-fly encryption for no cost still earns the freeware security tool a top rating.
The latest version gives users the ability to create a hidden operating system, encrypted away from nosy busybodies, but make no mistake--TrueCrypt is not for the casual encryption explorer. Online help is plentiful and detailed, but be sure to thoroughly understand what you're doing with the program before you do something regrettable.