Windows only: If you need to use both Outlook and Google Calendar, free sync utility KiGoo makes the process completely painless. KiGoo displays your Outlook and Google calendar side by side or overlays them to see all your appointments on a single calendar. Changes you make in either Google Calendar or Outlook appear quickly in both. You can manage your Google Calendar entirely from within Outlook if you desire—creating, modifying, and deleting appointments. The same synchronisation you get with your calendars applies to contact lists, too—KiGoo can move contacts between Outlook and Gmail or create a master list of contacts from both.
While it's nice to have both Google Calendar and Outlook in the same interface, the best use I got out of KiGoo was collaboration. Now my wife can push an event from her Google Calendar to mine and it appears instantly within Outlook (and consequently on my Windows Mobile phone). For another GCal syncing solution (sans contacts) check out Google's own Calendar sync application. KiGoo is a free download for Windows only, requires .NET 2.0 to run. Thanks, Nick!