I'm a sucker for great ideas with clever implementations involving an iPhone. This is no exception and it alleviates one of the most annoying things about working on a computer.
Don't want to have to type in your password every time you sit down at your computer but want to retain the security of a locked Mac? This literally solves the problem and utilizes low energy Bluetooth 4.0 to do it.
Mac app is free. iOS app (which currently only supports one Mac at a time) is $3.99. Just get within close proximity of your computer, pick up your phone, and knock on it. It will instantly unlock the computer.
Mac only, iOS only, for now. Cool video. Cool implementation.