SpamCheetah is a network-level spam filtering solution built around OpenBSD greylisting, tarpit, and blacklisting. It comes with a Web interface and requires zero administration. It does not allow spam to even land in your network, thus saving you precious bandwidth. It does not have the 'false positives' problem of losing legitimate mail.
Trojita is a Qt IMAP email client. It is a pure Qt4 application with no additional dependencies. It has a robust IMAP core implemented using Qt's Model-View framework. Standards compliance is a design goal. It has support for bandwidth-saving mode aimed at mobile users with expensive connections. In addition to the usual SSL/TLS connections, the server can be accessed via SSH. It has on-demand body part loading, offline IMAP support, and safe dealing with HTML mail (actually more robust than Thunderbird's).
tkpg is a program that provides a GUI front end to Philip Zimmermann's Pretty Good Privacy (PGP) public-key encryption system and also the GNU Privacy Guard (GnuPG/gpg) implementation. It utilizes the Clipboard Selection heavily to ease moving pgp-processed text both to and from any of your applications (such as mail reader, news reader, and editor). This removes the need to select the output text by hand or by typing in a filename, since it is usually placed in the Selection automatically for you.
qmailcheck looks for new mail in multiple IMAP mail boxes. If you get a lot of mail and split the messages into separate boxes (typically on the server side using procmail or sieve), then checking all the boxes by hand can be tedious. QMailcheck displays the results in a formatted list, highlighting the newest items. When new mail arrives in a folder, it is capable of raising the window, issuing a system notification, flashing the LED (on phones only), and vibrating (on phones only), all on a per-folder basis.
conv2047 is a very simple RFC 2047 Unicode email header field converter. It reads header lines from standard input and tries to decode or encode them. Operation modes: -d decode (MIME-Header --> UTF-8) and -e encode (UTF-8 --> MIME-Header). The -c option can concatenate multiple output lines into 1.
libquickmail is a library intended to give C/C++ developers a quick and easy way to send email from their applications. It supports multiple To/Cc/Bcc recipients and multiple MIME attachments without size limitation. It uses libcurl for SMTP communication, but a light version is also included without the libcurl dependancy. The library can also be used to just create a multipart MIME message body. It also comes with a command line application for sending mail (with optional attachments).
mu is a collection of utilities for maildirs, in particular for indexing and searching them. mu fills a database with information about your emails, which can then be queried using an easy query language. It can work stand-alone or be integrated with a mail client. It also provides an emacs-based email client ("mu4e") and bindings for Guile/Scheme.
Cura is a mobile phone application bundle of remote server administration tools. It provides a personalized terminal emulator, a syslog module that allows for reading logs directly from a server, a SysMonitor module that visually graphs CPU and RAM usage percentages, access to Nmap, and Server Stats will offer general server information like its Vitals, Hardware information, Memory information, processes, and so on. A security feature allows you to have Cura's database wiped when you send the compromised phone a secret pattern of your choosing. (e.g. send an SMS message containing "phone has been stolen!" to your Android phone to wipe Cura's database and receive the location of the compromised phone as an SMS to your emergency phone number or as an e-mail to your emergency email address).