ProtonMail - Secure email with absolutely no compromises. Get your free encrypted email account today.
Enigmail - Security extension to Mozilla Thunderbird and SeaMonkey
Skizzle Email - Skizzle is the most secure way to share end-to-end encrypted files through email, backed by blockchain
ProtonMail Extension - Unofficial webextension for ProtonMail
Tutanota - Get your encrypted mailbox for free.
GPGMail - GPGMail is an open source plugin for Apple Mail.