FullCalendar is a jQuery plugin that provides a full-sized, drag & drop calendar like the one below. 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). It is open source and dual licensed under the MIT or GPL Version 2 licenses.
FullCalendar is very flexible with tons of options. Except of your own events feed you can use Google or iCal calendars as your feed. This demo demonstrate Google US Holidays calendar as data feed.