Python CalDAV library. Combined with Google's CalDAV api it could be a powerful calendar backend.
Neat calendar service. Sync with desktop clients, mobiles, etc and then displays public availability and allows people to schedule events while handling timezones smoothly. Hat tip Anthony Feint.
Keep your mac synced with Google Contacts and Google Calendar. Yay for those Google loving mac using punks who want to use an Android device. Useless for us freedom lovers...
A drupal based intranet / collaboration platform type thing. Includes a collaborative calendar, document sharing, blog, micro-blogging, ticketing.
Google Calendar for CouchSurfing events in Sydney.
30 Boxes is an online calendar, todo and contact management system, a simple alternative to Google Calendar.