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.
jquery-calendar - A responsive jquery calendar scheduler built with bootstrap and moment.js
GTK - GTK+ is a multi-platform toolkit for creating graphical user interfaces.
Doodle - Make meetings happen. With Doodle, scheduling becomes quick and easy.
wxWidgets - wxWidgets: Cross-Platform GUI Library
Google Calendar - Spend less time managing your day & more time enjoying it