FullCalendar is a jQuery plugin that provides a full-sized, drag & drop calendar. It uses AJAX to fetch events on-the-fly for each month and is easily configured to use your own feed format (an extension is provided for Google Calendar). It is visually customizable and exposes hooks for user-triggered events (like clicking or dragging an event).
Source code and development has been moved to GitHub: http://github.com/arshaw/fullcalendar. However, the issues page is staying where it is for the time being.
Project homepage: http://fullcalendar.io/