Windows Outlook Users:
Calendar
- Google Sync
- OggSync
- gSync (this software also matches categories to Pocket Informant Calendars)
- Calgoo (free)
- SyncMyCal
Mac users:
Calendar
- iCal supports Google's CALDAV directly
- Google Colaboration
- Calgoo (free)
- SpanningSync(no alarm sync support)
- BusySync
- NuevaSync
Windows/Linux/Mac SunBird/iCal/etc... Calendar