Android: Void Lock keeps your phone secure by turning off your screen, as if the phone weren't on at all, and letting you unlock with a gesture. If you really want to keep someone out of your phone, Void Lock has a very cool method for doing so: instead of presenting you with a gesture unlock screen, the screen stays turned off.
Even if you press the power button on your phone or try and turn it off, your phone will appear as if its dead, until you swipe your gesture on the seemingly off screen. It's a very clever lock and works remarkably well on the phones it supports. You can even set other gestures to launch apps, change settings (like Bluetooth, Wi-Fi, or vibration) and set a failsafe button combination (like Power, Volume Up, Volume Down, Volume Up) in case something goes wrong.
Void lock is available as a free, ad-supported app or for $3.99 without ads. Right now, it only works on non-Ice Cream Sandwich phones and has issues on phones with AMOLED screens (though the developer is working on both issues for the next release). Hit the link below to check it out.