For women who have regular or only slightly irregular cycles, there are many websites out to help with Calendar Rhythm Method. Of course, they are promoted primarily for couples trying to conceive.
But BabyCenter.org has taken its charting page to a new level by offering a text messaging service. It requires a knowledge of the average cycle length, but you enter the information here, and it estimates the most fertile time. Enter a cell phone number, and it sends 3 text messages during the days that should be peak, for 6 months.
The Lewis Crusade 