Kleopatra - Kleopatra is a certificate manager and GUI for GnuPG.
Toolsley PGPigeon - Simple but powerful OpenPGP-compatible message encryption webapp.
Seahorse - Front end for GnuPG.
GPGTools - Use GPG Suite to encrypt, decrypt, sign and verify files or messages.
GnuPG - GnuPG is a complete and free implementation of the OpenPGP standard as defined by RFC4880 (also known as PGP).
Lockbin - Lockbin is a web application for sending private email messages and files.