Knock is an iPhone app that allows you to unlock you Mac by simply knocking your iphone. You will also need to install their Mac app available for free on their site.

The iPhone app is currently on special at $3.99 on the appstore.

Installation takes a couple of minutes and voila. The iPhone app has to be started but doesn't have to be the active app when knocking it. (and your phone can even be locked).

This black magic uses low power Bluetooth and the transaction is done over a secure connection.

The company     is working  on a Windows version as well.