GPGMail - GPGMail is an open source plugin for Apple Mail.
Knox - Knox offers easy disk image encryption and comes with a built-in backup service.
ProtonMail - Secure email with absolutely no compromises. Get your free encrypted email account today.
Advanced File Security Basic - Download Advanced File Security Basic. Protect files by encrypting them with a strong password, a series of questions, or even an USB key you can carry around with this powerful security solution
Enigmail - Security extension to Mozilla Thunderbird and SeaMonkey
Exocet - Exocet creates a Folder on your System.