Are you female? Swell! Do you like to break things? Even better! You should try to break my beta web app so I can fix bugs.
You can find a description and screenshots here. In brief:
Mon.thly.Info is a simple tool to help you keep track of your menstrual cycles.
Each time you start your period, add the date to your Mon.thly account, and it will use your history to predict the next time your cycle will start. If you want, Mon.thly will also email you a customized reminder before or on your next estimated start date.
(Of course, the fellas are also welcome to break it if they are curious.)
http://mon.thly.info/