TimeAndDate (http://www.timeanddate.com/)

Kids with friends or relatives in other lands will love the Meeting Planner. With it, you can find a time for your chat when everyone is likely to be awake, even though the participants may be far-flung across several time zones. And don't miss the calendar section. Want a calendar for the month you were born? Just type in the relevant date and select size, type of holidays to display, and whether you want the moon phases to show.

Another fun thing to do is calculate how many hours, minutes, and seconds old you are, or try the countdown to New Year's, or a countdown to a custom date you can set yourself.