mpop is a small, fast, and portable POP3 client. Its features include header-based email filtering (filter junk mail before downloading it), delivery to mbox files, maildir folders, or a mail delivery agent, a very fast POP3 implementation, many authentication methods, and good support for TLS/SSL.
mrtoms is an IRC bot written in Scala. It supports a very powerful and easy to use plugin system where plugins are compiled to and loaded from jar files. mrtoms comes with a Z-machine interpreter plugin so that games like Zork can be played collaboratively. It also includes an MPD control plugin and others.
mru_manager is a PHP module to access the most recently used items in a Web application. It can register and store "opening object" events. It can manage the order of events and delete all events that exceed the configured maximum number for each object type. The list of recently used items can be saved with user credentials. Separate lists can be maintained for different types of objects.
msg-rpc provides bidirectional RPC support over a simple message interface such as WebSocket. Tere are WebSocket examples based on socket.io and sockjs. It just has a simple sendMessage/onMessage interface, with no other dependencies. It supports not only normal RPC, but also RPC Service, which makes RPC pushable (i.e. the server can also send a request to the client and expect some kind of response).
msmtp is an SMTP client with a sendmail compatible interface. It can be used with Mutt and other MUAs. It forwards messages to an SMTP server which does the delivery. Features include various SMTP AUTH methods, TLS/SSL-encrypted connections (including support for client certificates), support for multiple accounts, DSN, and IPv6 support.
mtPaint is a simple GTK+1/2 painting program designed for creating icons and pixel-based artwork. It can edit indexed palette or 24 bit RGB images and offers basic painting and palette manipulation tools. Its main file format is PNG, although it can also handle JPEG, GIF, TIFF, BMP, TGA, XPM, and XBM files.