Unlock my (#$&(#$ iMac, OK?
Take out the cursing and that's the idea behind Knock, a pair of apps (free for Mac, US$3.99 for iOS) that lets you unlock your Mac by simply knocking twice on your iPhone. You don't need to even take the iPhone out of your pocket -- rapping twice on the device, even through your favorite jeans, types in your password and unlocks your Mac. No more sitting down and typing your 11-character safe password -- just knock twice.
The app works on just about all new Macs -- the 2011 or newer MacBook Air or Mac mini, the 2013 or newer MacBook Pro, and the late-2012 or newer iMac (note that the iTunes app description says 2013 or newer, but it works perfectly on my late-2012 iMac). The "key" app that is installed on your iPhone works on an iPhone 4s or newer that's equipped to handle Bluetooth LE (Low Energy).
It's that use of Bluetooth LE that lets this work without draining your iPhone battery. Your iPhone and Mac will be linked whenever you get close to the Mac with your phone, and that double-rap sends a signal to the Mac to "open up." The go-ahead to unlock from the iPhone app is encrypted via 2,048-bit RSA encryption, and Knock Software notes that the Mac app "makes sure your Mac's lock screen isn't being manipulated by bad guys" before unlocking, although they smartly don't say how or why.
So, does it work? Heck, yeah. And considering that I unlock my iMac screen about 10 times a day with an 11-character password, being able to do this with a knock is like magic. I figure that Knock will pay for itself very quickly in terms of saved time.