VeraCrypt - VeraCrypt is a free open source disk encryption software for Windows, Mac OSX and Linux.
CrococryptFile - Strong file encryption tool
Easy File Locker - Easy File Locker is a pretty small file at 224KB and will do exactly what it says.
GPG Suite - Use GPG Suite to encrypt, decrypt, sign and verify files or messages.
Axcrypt - AxCrypt - Password Protect Files With Strong Encryption. AxCrypt is the leading open source file encryption software for Windows.
GnuPG - GnuPG is a complete and free implementation of the OpenPGP standard as defined by RFC4880 (also known as PGP).