Postmaster is an Internet mail transport agent (MTA) that handles incoming ESMTP network connections and delivers accepted messages to the user's mailbox by piping it into an arbitrary local mailer (such as Procmail). It is configurable beyond anything you'll ever need, handles any number of concurrent connections without forking or using OS threading, and it is pretty efficient. It currently lacks any form of queue management, but for leaf sites that don't need to do extensive mail relaying, it is a reliable and powerful solution.
twidge is a client for microblogging sites such as Twitter and Identica (identi.ca). It is extremely versatile, can be customized to work the way you want to work, and can be combined with other tools to do just about anything. It can be used interactively from the shell. It is useful directly as-is, and with simple shell aliases can make a highly efficient system to do exactly what you want. It is perfectly capable of being your only client for microblogging. It can also be used in an automated way, via cron(1), or it can even integrate with your email system.