WebCalendar is an online multi-user event scheduling and day planner system. Users can create accounts, login, add events to their calendars, and share events with other users. Support for different time zones is also major feature, and allows users to view events in their local time, regardless of what time zone they may have been added from.
I started the WebCalendar project in September 2004 as a way of teaching myself some PHP and database skills. Since then, through use in several school projects, I have enhanced WebCalendar to the point where it is at least semi-useful as a personal organizational tool.
Below you can see some screenshots of WebCalendar. You can also download the source code and try it yourself (although I don't really recommend using it; there are plenty of better options out there).