Keycard is undoubtedly the easiest way to keep your Mac secure when you're not around.

Using Bluetooth, Keycard locks your Mac using your iOS device when it detects you are leaving your computer. When you come back, it unlocks it for you. Leaving your desk for 10 minutes? Grabbing a coffee in the library? Take your iPhone or other iOS device with you and it can serve as your own personal "Keycard."