Android: There's no shortage of lockscreens for Android, but Picture Password Lockscreen (inspired by a similar feature in Windows 8) may be one to end them all. The app replaces the traditional pattern or swipe-to-unlock screen with a static image, and requires you to draw symbols on a background image that you select in order to unlock your phone. You set the pattern, and only you know how to draw the symbols correctly.
The beauty of Picture Password is that it asks you to draw a dot, a line, and a circle anywhere on a background image before the phone will unlock. When you walk through the setup wizard, you draw those symbols anywhere on the screen you want, and the only way to unlock the phone is to draw them the way you did when you set up the phone. Even if someone watched you do it, they'd have a hard time duplicating the pattern, and there are no on-screen clues to help a stranger or thief unlock your device.
When you set up the app, you have the option to enter a backup PIN and set how many retries you get before you're prompted for it. We strongly recommend you do so, just in case. When I tested the app, it was a little quirky on my Droid Bionic, but it eventually worked smoothly. The video above from XDA-Developer TV shows you how the app works (skip forward to the two-minute mark to see it in action,) and the app is completely free at Google Play and adds a great layer of security to your phone. If you like the app, there's a "Plus Key" for $3.43 that adds the time to the lock screen, the ability to start the app on boot, and a few other features.