Well, just for the sake of trying something, I went into the Play store and searched 'period tracker' -- I went ahead and downloaded Period Tracker since it had decent reviews. Verdict on this one so far after poking at it for just a few minutes: functionality is pretty limited in the free version but appears to cover the basics, the UI has pretty illustrations but is a bit cluttery and I don't like the date input -- I shouldn't have to scroll a wheel, I should be able to just type in a date. I may download some others to mess with, I'll let you know. :)