PHP Event Calendar - PHP event calendar script with multiple views and many customizing options.
Qt - Powerful, flexible and easy to use, Qt will help you not only meet your tight deadline, but also reduce the maintainable code by an astonishing percentage.
Doodle - Make meetings happen. With Doodle, scheduling becomes quick and easy.
GTK - GTK+ is a multi-platform toolkit for creating graphical user interfaces.
Yahoo! Calendar - You can view an entire month or focus on a specific date and time.
wxWidgets - wxWidgets: Cross-Platform GUI Library