This article demonstrates how easy it is for others to snoop around in your personal affairs if you like to use "free" services like Google Calendar. The author, simply by clicking on calendar items and using the information to dig up additional detail, was quickly able to identify where a woman lived and when she would be out of the house (handy for burglars).
Google Calendar has a privacy setting, but not everyone uses it. Sharing a calendar with your friends and family often has the unintended side effect of sharing it with everyone else in the world. Use these services carefully, and never, ever let your children use these.