9. Contact Spy
We all know rule number one of sneaking around is you never put your mistress's contact in your phone. If you're real slick, you'll put the other woman's number in your address book under a different name. When she calls, you can play it off as a work thing or a wrong number. With the Android and iPhone app Contact Spy, if your lady catches that "work number" hitting you up six times on a Friday night, she can plug it into her phone and quickly learn that it wasn't Steve from accounting calling after all.