Android: Android has plenty of ways to make your lock screen more functional and secure, but once that's bypassed, there's no built-in ability to lock individual apps. App Lock (Smart App Protector) is the best app I've seen to fill this void.
It packs an impressive list of features. You can lock apps by password (character or number), pattern or gesture, and use that to lock your whole phone too. There's also an option to set up multiple passwords and assign them to apps, so you can have different passwords for different apps. It's pretty easy to set up.
One of the cool features is that App Lock can fake out someone who is looking to access an app. If a stranger were to tap a locked app, it can fake Android's default prompt "Unfortunately, <app name> has stopped working." Tap OK and you'll be returned to the home screen. But hold down OK and you'll be taken to the password window, where you can unlock the app.
Additionally, App Lock also features the ability to take photos of snoops, much like PeeperPeeper -- and does a better job of the fake-out too. You can even lock core settings, like 3G or Wi-Fi toggles. In fact, the only thing App Lock is missing is the ability to unlock with face detection, for which you'll still need to rely on an app like Visidion.
App Lock (Smart App Protector) (Free) [Google Play Store]