libpubsub provides everything needed to set up a simple publish/subscribe environment. It allows a programmer to let several processes (not necessarily on the same host) exchange information with each other in a very easy way. It is mainly intended for use in C++ applications, but the package also includes a tool to let shell scripts use pubsub.
sms-queue handles an SMS queue for small networks or single user machines. It is designed for areas where many SMS messages are sent. It does not actually send the SMS itself, but instead calls an external application (depending on the SMS gateway you want to use) via a simple standard interface. The communication between client and server is based on a simple ASCII protocol over TCP/IP which should make it easy to develop your own frontend.
Mahogany is a GUI email client for the X11/Unix, Windows, and Mac platforms. It supports a wide range of Internet protocols (IMAP4, POP3, ESMTP, NNTP) and encrypted communications using SSL, includes full MIME support, and aims to be fully standard-compliant. It allows the use of unlimited number of servers, supports filters and identities, and offers optional HTML viewing and much more.