Appealing to females who want to keep track of their menstrual cycles, Period Pace [FREE] is most definitely a top-contender. Why? Its easy-to-use interface and features that give you the best estimates make it an ideal application, especially considering it's absolutely free to download.
Coming from iPhone developer Aesop, who specializes in the making of applications appealing mostly to just-married couples, and helping with care of a newborn child, Period Pace certainly is the solution to tracking your periods.