GPGMail - GPGMail is an open source plugin for Apple Mail.
HEY - Email at its best, new from Basecamp.
ProtonMail - Secure email with absolutely no compromises. Get your free encrypted email account today.
Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
Enigmail - Security extension to Mozilla Thunderbird and SeaMonkey
InstAddr - Disposable email address application.