VeraCrypt - VeraCrypt is a free open source disk encryption software for Windows, Mac OSX and Linux.
GnuPG - GnuPG is a complete and free implementation of the OpenPGP standard as defined by RFC4880 (also known as PGP).
Kleopatra - Kleopatra is a certificate manager and GUI for GnuPG.
Windows BitLocker - This topic provides a high-level overview of BitLocker, including a list of system requirements, practical applications, and deprecated features.
PGP Tool - PGPTool allows you to encrypt and decrypt files. It's free and easy to use.
TrueCrypt - TrueCrypt is a leading cross-platform that provides encryption tools that allow you to migrate any encrypted data to virtual disks images on your systems.