Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
GnuPG - GnuPG is a complete and free implementation of the OpenPGP standard as defined by RFC4880 (also known as PGP).
Roundcube - Web-based IMAP email client
Kleopatra - Kleopatra is a certificate manager and GUI for GnuPG.
Mailo - Mailo is an email client where you can send and receive emails to and from anyone with an email address.
PGP Tool - PGPTool allows you to encrypt and decrypt files. It's free and easy to use.