VTCalendar is a web-based event calendar that allows for decentralized data input and includes an approval mechanism. It supports calendar hosting for individual organizations and the forwarding of events from these calendars to the main event calendar.
I am reviewing VT Calendar 2.3.0, which has been the latest release for several years now. We have been using this software heavily in a corporate environment for five years. I have found that all the basic functionality we need is there, and the database structure and code are clean enough that we have been able to make modest enhancements easily. We have modified the authentication (basically turning it off, we authenticate elsewhere) and have added some minor bits to the event editor to make it a little easier to add URLs, etc. I have posted these to the project website as bugs/enhancements so you can see the code there. We have also written a small perl script to extract the latest 3 days and format it for our intranet home page. From there all the links go to the stock VTCalendar month/week views. But that's about it for customization. VTCalendar does everything we need, and it's written cleanly enough that you can tweak it if you want. That's all I ask from a piece of software!