Apple iCal and Mozilla Sunbird
You can now sync your calendar with applications that support CalDAV, such as Apple's iCal and Mozilla's Sunbird. Learn more about how to get started in our help center.
Google Calendar Sync for Outlook lets you sync events between Google Calendar and Microsoft Outlook Calendar. You determine the direction of information flow, as well as the sync frequency. It's compatible with Microsoft Outlook versions 2003 and 2007, and operating systems Windows XP and Windows Vista.
BlackBerry mobile devices
If you carry a BlackBerry smartphone, Google Sync for BlackBerry synchronizes your device's native calendar with Google. You can access your Google Calendar even when you don't have network coverage and get alerts for upcoming appointments with sound or vibration. It automatically runs in the background, so you can attend to other tasks and still be reminded of your appointments.
It's easy to connect an Android-powered phone to Google Calendar using the phone's preloaded Calendar application. New events are pushed in real-time to your phone and any changes you make on-the-go are immediately available on the web.
If you have another mobile device, just visit calendar.google.com from your phone, and you'll get a special version of Google Calendar optimized for the small screen.
[NFGB] Link - from Official Gmail Blog