Compact Header - Stylish script that gives you more space by removing the header of Google Calendar.
Extra Tabs - User script that changes the custom date range tab to multiple tabs of your choice.
Full Cal - Add-on gives you a larger view activated by a customizable hotkey.
Google Air Skin - Greasemonkey hack to give Google Calendar, Gmail, Google.com, and Google Reader a nice skin.
Google Calendar Optimized - User style that compacts the paddings and margins and lowers the font size.
Header and Navigation Switcher - Hide the navigation and header bars with this user script.
Show Week - Greasemonkey addon that adds week numbers.
Slate Series - Grey user style skin, ideal for those depressive autumn days.
Textwrap Events - Wrap long event names so they're actually readable.
Wheel View - Script to let you move the calendar with your mousewheel.
Add Your Agenda To Gmail - Tutorial on adding your Google Calendar agenda as a Gmail sidebar.
Facebook to Google Calendar - Greasemonkey script to send Facebook events to Google Calendar.
Google Calendar Forecast - Adds your agenda to every webpage you visit.
Google Calendar Widget For TypePad - Put your calendar in a sidebar for your TypePad blog.
Howto integrate Google Calendar in you website using AJAX - Short tutorial that shows you how to put your agenda into your website.
iGoogle Calendar Gadget - A simple Google Calendar gadget for your iGoogle homepage.
Remember the Milk For Google Calendar - Add your RTM to-do list to Google Calendar.
WoogleCal - Adds Google Calendar to your Wordpress blog.
Calgoo - Java app that syncs Google Calendar, iCal, and Outlook (and works as a standalone app itself).
GCALDaemon - Google Calendar synchronization for virtually any iCal-compliant client. Also includes a Gmail notifier, a Gmail terminal, an RSS>iCal converter, and much more. Tutorials here and here.
Gcal.app - Minimalistic WebKit browser for OS X that displays Google Calendar.
GDATA Provider - Allows read/write access to GCal from Lightning/Thunderbird and Sunbird (tutorial here).
Google Calendar Gadget - Google Desktop plugin that displays your calendar.
Google Calendar Mobile - Google provides a handy mobile version of your calendar. Just go to calendar.google.com in your phone, and it will redirect you to a special mobile version.
Google Calendar Quick Add - Firefox add-on to quickly create an event.
Google Calendar Widget - Yahoo Widget Engine (formerly Konfabulator) widget that displays your calendars on your desktop.
How to make your Google Calendar your PC's desktop wallpaper - Lifehacker tip on making Google Calendar your wallpaper in Windows XP.
Milo Calendar - SuperKaramba widget for KDE that views your calendar on your desktop.
Rainlendar - Windows/Linux app that displays Google Calendar on your desktop.
Remote Calendars - COM-.NET Add-in for Outlook 2003 that has two-way sync with any iCal calendar (including Google Calendar, see here for more).
SyncMyCal - Sync Google Calendar with Outlook XP/2003/2007.
Twittercal - Create events by sending messages to the gcal bot on Twitter.
Google Calendar Notifier - Firefox add-on with easy notification for today (and tomorrow)'s agenda.
Google Notifier for Mac - Get Gmail and Google Calendar notifications with this handy app.
IMified - Add events and view upcoming tasks through AIM, Yahoo! Messenger, MSN, Jabber, or Google Talk.
Better GCal - A Firefox extension that adds the Air Skin, Collapse Header and Sidebar, Colorize Weekends, Force Secure Connection, Show Week, Slate Skin, and Text Wrap Events to Google Calendar.
GCalSecure - Makes GCal use a secure connection.
Google Calendar Quick Import - Greasemonkey script to import ICS/iCal events into Google Calendar.
hCalendar to Google Calendar - Add hCalendar events to Google Calendar with this handy user script
todo list prototype -.Adds a to-do list to Google Calendar (and stores entries in your local gm_scripts folder).
TODOlist - Improves the above script to save events to Google Calendar. (via)