Mailvelope - Mailvelope open source project. OpenPGP in your browser.
Exocet - Exocet creates a Folder on your System.
ProtonMail Extension - Unofficial webextension for ProtonMail
Kruptos - Lock and encrypt your photos, movies, files and folders using 256-bit file encryption.
GPGMail - GPGMail is an open source plugin for Apple Mail.
R-Crypto - R-Crypto is smart data security and disk encryption software that allows you to secure your sensitive information via strong encryption.