iPhone: If you're worried that someone might be snooping on your phone when you leave it unattended, Catchr is an app that allows you to see what apps were opened and closed when you left your phone sitting around.
Catchr records which apps you open and close and whether or not your phone has been moved. So, if you leave your phone unattended on a table, and your friend decides to poke into your text messages, you'll see that it was opened and how long it was open for. You'll need to close all your apps out for Catchr to really work that well, but it still makes it easy to track whether someone's using your phone when you're not around. Sure, a passcode is better for protection, but Catchr lets you catch them in the act.
Catchr ($1.99) [iTunes App Store]