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).
Easy File Locker - Easy File Locker is a pretty small file at 224KB and will do exactly what it says.
Kleopatra - Kleopatra is a certificate manager and GUI for GnuPG.
Axcrypt - AxCrypt - Password Protect Files With Strong Encryption. AxCrypt is the leading open source file encryption software for Windows.
PGP Tool - PGPTool allows you to encrypt and decrypt files. It's free and easy to use.