This one goes out to all the ladies: calculate the start-date of your period for the next 12 months!
Ladies! Girls! Husbands or significant others of girls and ladies! Thanks to the work of plucky teenager Tina Marsh, you can now calculate the starting date of your period for the next twelve months. Just plug in the date of your last period, your usual cycle length and click 'calculate'. Voilá -- a yearly calendar with 'P' illustrating which day of the month your period will start.
If that wasn't cool enough, you can also export the dates in CSV format. From there, you can import your periods into Microsoft Outlook, Google Calendar, iCal... etc!
Finally, purely from an educational standpoint, there's a cool Flash animation on the Period Calculator homepage that teaches you all about the menstrual cycle. Whether you're an intrigued male, or a lady that wants to know more about their bits, it's worth watching!