I used track my cycle by noting the first day of my cycle and doing the average by hand every so often, but I wouldn't always remember to write the date down. Then I tried downloading an app last spring, I don't remember what it was called but it was awful! Difficult to use and really ugly pink and purple interface. Then I found a better one that I've been using for a while, and I wanted to share here if anyone's interested: http://www.helloclue.com/app.html
I think it's pretty easy to input data and it calculates your average cycle for you. The main screen shows you at a glance where you are in your current cycle, and there's a screen for history where it shows you your past cycles. So now I don't have to awkwardly try to count back the days in my head trying to figure out when my period will show up, I can open the app and see that it's estimated for next Tuesday, which is super helpful. There's a bunch of other features like tracking PMS, family planning stuff, and a bunch of options for reminders, but I don't use most of them.