A free app for Android, by Cometstack.

Invisiblock is an app locker that provides a unique, invisible gesture-unlocking mechanism.

This is the best way to protect your privacy when using your phone.

You can select the apps you want to lock and set a secret gesture for unlocking.

When someone else accesses your phone and tries to open a locked app, a fake error screen would come up, and he/she would have no idea what happened!

The only way to gain access is to draw the secret gesture over the fake error screen.

Note that the gesture drawn would be invisible, and hence nobody would be able to know its existence.