GPGMail - GPGMail is an open source plugin for Apple Mail.
VeraCrypt - VeraCrypt is a free open source disk encryption software for Windows, Mac OSX and Linux.
ProtonMail - Secure email with absolutely no compromises. Get your free encrypted email account today.
Axcrypt - AxCrypt - Password Protect Files With Strong Encryption. AxCrypt is the leading open source file encryption software for Windows.
Enigmail - Security extension to Mozilla Thunderbird and SeaMonkey
Windows BitLocker - This topic provides a high-level overview of BitLocker, including a list of system requirements, practical applications, and deprecated features.