Kayla introduced me to this site MonthlyInfo.com. It is AMAZING. It's simple, easy, free, and for some reason makes relieved and at peace with my period. I don't want to sound like a paid advertiser or anything, but everyone with a period should use this!
Basically, you just put in when you get your period on the calendar.
Then, from the day it starts it will show you what stage you're at, whether it's a maturing follicle, ovulation, etc. This would be great for any trying to do natural family planning as well, so you can see when you're ovulating or not.
There is also a place on the calendar where you can write notes about your period. Then, as you record it more often, you can start to see your cycle pattern. Obviously when you first start it, it's hard to tell what your pattern is completely accurate. As I've been using it for a few years now, I can look through and see the variations of my cycle durations. There is also a chart showing the average durations and the standard deviations of them too!
My favorite part about this site is that it will send you friendly reminders when you're period is expected to come. You can sign up for emails or texts from them to let you know. I have mine signed up to text and email me two days before it's expected.
And that's it! All you do to sign up is give your email and make a password. I love the layout of it because it's simple and not evil and scary, like our periods usually seem to be. So I suggest if you have a period, you should have a MonthlyInfo account. I seriously just love it. It's great to see when my cycles are normal or off, and getting little reminders of when it'll be. :)
[And I will step off my soap box now]