John the Ripper is a fast password cracker, currently available for many flavors of Unix, Windows, DOS, BeOS, and OpenVMS. Its primary purpose is to detect weak Unix passwords. It supports several crypt(3) password hash types commonly found on Unix systems, as well as Windows LM hashes. On top of this, lots of other hashes and ciphers are added in the community-enhanced version (-jumbo), and some are added in John the Ripper Pro.
Pine Privacy Guard is a small Perl script used to interface Pine and GnuPG for the secure exchange of email. It allows Pine users to conveniently encrypt, decrypt, clear-sign, and verify clear-signed email. Other features include the secure storage of secret key passphrases during a session, alias support, simple installation, and compact, well-documented code.
RFC 2015 defines a proposed Internet standard for sending PGP-encrypted email. This PGP/MIME has been incorporated into several MUAs such as Mutt and (with the AEGYPTEN project) KMail. However, a lot of email clients still don't support it. pgp-mime-handler can decrypt/verify such messages via a pipe, so it can be used as an email filter in many programs or scripts.
GNU Anubis is an SMTP message submission daemon. It represents an intermediate layer between the mail user agent (MUA) and mail transport agent (MTA), receiving messages from the MUA, applying to them a set of predefined changes, and finally inserting modified messages into an MTA routing network. The set of changes applied to a message is configurable on a system-wide and per-user basis. The built-in configuration language used for defining sets of changes allows for considerable flexibility and is easily extensible.
Tiger Envelopes makes encryption automatic with almost any email program, allowing you to continue reading and writing mail the way you always have. It installs with one click on Windows, Mac, and Linux. It automatically configures Outlook, Outlook Express, Thunderbird, Mac Mail, and KMail. Tiger is a personal mail proxy server with automatic encryption. It uses POP3, IMAP, and SMTP to work with almost any mail client, crypto plugins to work with almost any type of cryptography, and Java to run on almost any operating system. Plugins for Bouncy Castle, GPG, and PGP are included.
Enigmail is an extension for the mail client of Mozilla, Netscape 7.x, and Mozilla Thunderbird that allows users to access the authentication and encryption features provided by the popular GnuPG software. It can encrypt/sign mail when sending, decrypt/authenticate received mail, and import/export public keys. It supports both the inline PGP format and the PGP/MIME format, which can be used to encrypt attachments, and is cross-platform, although binaries are supplied only for a limited number of platforms. Enigmail uses inter-process communication to execute GPG to carry out encryption/authentication.