Google Calendar

• Seeing the big picture
  With Google Calendar, you can see your friends' and family's schedules right next to your own; quickly add events mentioned in Gmail conversations or saved in other calendar applications; and add other interesting events that you find online.

• Sharing events and calendars
  You decide who can see your calendar and which details they can view. Planning an event? You can create invitations, send reminders and keep track of RSVPs right inside Google Calendar. Organizations can promote events, too.

• Staying on schedule
  You can set up automatic event reminders, including SMS notifications, and instantly bring up anything on your calendar with the built-in search tool.

http://www.google.com/calendar/