Google Calendar added “CalDAV support”, which is an open protocol that allows calendar access via WebDAV. CalDAV models calendar events as HTTP resources in iCalendar format, and models calendars containing events as WebDAV collections. This allows you to publish and subscribe to calendars, share them collaboratively, sync between multiple users and sync between multiple devices.
For now, the only application supported by Google Calendar is Apple's iCal. “With CalDAV support in Google Calendar, you'll be able to view and edit your Google Calendar events directly in iCal. Any changes you make in iCal will automatically appear in Google Calendar the next time you sign in (and vice versa). If you use iCal while offline, changes you make will be saved and updated in Google Calendar when you get back online.”
Google Calendar also provides basic applications for synchronizing data with Blackberry devices and Outlook, but there are many third-party applications that use Google's API to add more advanced functionality: Spanning Sync (iCal - $25/year), BusySync (iCal - $25), Calgoo (iCal/Outlook - free), SyncMyCal (Outlook - $25), gSyncit (Outlook - $10), GCALDaemon (cross-platform/open source), Plaxo (cross-platform/free), GCalSync (Java mobile app - open source), GooSync (SyncML service/mobile app - £20/year), GMobileSync (Windows Mobile - open source), OggSync (Windows Mobile/Outlook - $30/year).
Source:→ Google System