Microbrew Message Center is designed to be an immensely scalable mail platform supporting multiple users, multiple domains, and multiple mail servers. It is akin to iPlanet or post.office but is not as complex and uses open source software to perform mundane services such as Web and mail services.
Drivel is a GNOME client for working with online journals, also known as weblogs or simply blogs. It retains a simple and elegant design while providing many powerful features, including support for LiveJournal, Blogger, MovableType, Advogato, and Atom journals. (Systems based off these are also supported, including WordPress and Drupal.)
LineControlTime is a server which allows you to control which users can connect to the Internet through your server. It is based on Stefan Fuchs's LineControl, and it features a 'Time Control' feature which uses a MySQL database to store the amount of time left for each user. The authentication mechanism is based on PAM. Users can also be authorized or refused according to the current time. It works with ISDN or analog lines.
QPlot is an Advanced Matematical Calculator for Qtopia, featuring a nice calculation mode (you see the statement and the result), arbitrary precision calculations, an extensive math library (trig, inverse trig, hyp, inverse hyp, algebraic, etc.), integration, differentiation, quadratic differentiation, graphing (including the ability to easily move around and zoom in/out), equation solving (quadratically or using Newton's method),an ability to write programs (and run them directly from desktop), and extensibility.