Android: Last month, we got news of a particularly nasty Android vulnerability called Stagefright that affects nearly every Android device. If you want to find out if you're vulnerable, Stagefright Detector can let you know.
The app comes from the same security company that first discovered the bug. Of course, this app doesn't actually fix anything. For that you'll need to wait for a security update to roll out to your device. The severity of this bug has prompted Samsung and Google to start a monthly security update program, though we're still waiting to see if the rest of the industry as a whole will work together to make security updates like this easier.