Kleopatra - Kleopatra is a certificate manager and GUI for GnuPG.
Toolsley PGPigeon - Simple but powerful OpenPGP-compatible message encryption webapp.
Enigmail - Security extension to Mozilla Thunderbird and SeaMonkey
DeliverySlip - DeliverySlip helps simplify, secure, and expedite the burdensome elements of online collaboration...
OpenKeychain - OpenKeychain helps you communicate more privately and securely.
Lockbin - Lockbin is a web application for sending private email messages and files.