Mac: HandyLock is a utility that makes sure your Mac is locked and password-protected every time you get up and wander away from it. It uses your phone -- or any other Bluetooth device -- as a "key". When it's out of range, the app will lock your screen for you, automatically.
At my last job, it was common for people to prank each other when they left their computers unlocked -- an embarrassing email to the entire team or a new wallpaper. It was all in good fun, but it's important to remember to lock your computer with a password anytime you get up and walk away. If you're short on memory, or you just want an app to do it for you, HandyLock is perfect. Download the app, give it a password (it can be used in conjunction with your default screensaver or password protection), and then set up Bluetooth from inside the app.
The video above, from our friends at Tekzilla shows you how it's done. Any Bluetooth device will work, and you can select it from the built-in menu. Pair it, and the app will ask you to place it where it normally lives, either on your desk or in your pocket, so it can calibrate itself. Then, you can play with the sliders to determine when HandyLock will auto-lock your computer based on Bluetooth signal strength. You can even set the app to start on login and hide itself so people won't just disable it.
Once it's all set up, just run it, walk away from your computer, and watch it lock itself. Walk back, and it will unlock so you can get back to work. HandyLock is free, and you can download it at the link below.