While trying to conceive, one of the first things my wife started to do was calculate her ovulation date. Now by the time I wrote this app, it was too late to be useful for us, but it still offers a simple calculator that can be used to figure out one's estimated ovulation date.
Now, I would suspect that this would be used to help get pregnant, but alas my friend Jamie informed me it could be useful in the opposite direction too ;)
You can find the calculator here on the iTunes store.