VTCalendar is a web-based event calendar originally developed in an effort to reduce the number of event calendars at Virginia Tech.
It allows for decentralized event input and includes an approval mechanism. It also supports calendar hosting for individual departments and organizations, and allows them to forward their events to the main event calendar.
VTCalendar is designed with universities and large colleges in mind. However, as software offered under GNU LGPL, you are free to customize it for your environment.