Submitted by Kelsey (thanks!):
I use a nifty iPhone App called Period Tracker Deluxe (it's also available as a free version, called Period Tracker Lite.) It's essentially genius, and lets women track the length of their periods (and the projected start dates of their next period), their symptoms, moods, cravings, weight, basal body temperature, consistency of their discharge, and through a series of charts and graphs also shows them their most fertile window and when they're due to ovulate. It also lets them keep track of when they're intimate, and, as such, when they have more reason to panic when their period is MIA. Mostly I find it useful to recognize patterns in my cycle, as way of getting to know my body overall.
This is the website for it: link.
I started out with the P-Tracker Lite version, but decided to splurge the $1.99 and upgrade. I'm sure there are billions of apps out there that do this, but I figured I'd share my success story.