Open PGP and PGP / MIME Email Software

Balsa
Carve your emails in Balsa with this nice and quite powerful email client. (Linux/BSD/Unix)

Enigmail
Enigmail adds easy to use and competent support for both plain text and MIME-compatible OpenPGP message encryption and signing to Mozilla, Thunderbird and Netscape. (Windows, Linux, Mac)

GPGMail
GPGMail adds strong OpenPGP/MIME email encryption to Mac OS X Mail. (Mac)

marlin
marlin is a nice email client with a convincing security focus and a solid assortment of features. (Windows)

PGP for Pegasus Mail
PGP for Pegasus Mail adds flexible and powerful PGP and OpenPGP message security to Pegasus Mail. (Windows)

PGP Mail for Windows
PGP Mail provides strong public key encryption with a comfortable interface and easy to use plug-ins for popular email clients. (Windows)

The Bat!
The Bat! is a highly sophisticated and efficient email client. It makes handling even large volumes of mail a pleasant and secure experience. (Windows)

Ximian Evolution
If Linux is better than Windows then Ximian Evolution is better than Outlook. Linux is better than Windows. (Linux/BSD/Unix)