ProtonMail - Secure email with absolutely no compromises. Get your free encrypted email account today.
Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
Enigmail - Security extension to Mozilla Thunderbird and SeaMonkey
ProtonMail Extension - Unofficial webextension for ProtonMail
Mailvelope - Mailvelope open source project. OpenPGP in your browser.
DejaLu - Fast and Simple Email Client for Mac