Which Shared Calendar Package Would You Use?

Bob McCown asks: "I manage several websites, both internally and externally accessible. Many of them have event calendars or schedulers. We'd like the ability to have these calendars shared, with the ability to modify them by both a web interface, and at the application level (via Sunbird, an Outlook plugin, or something similar). The web side of our system uses an Enterprise Linux distribution that runs Apache. Ideally, the web side would be written in PHP to minimize time to integrate with the rest of the sites. What's out there that can do this? What have you used before?"