Knock is a new app available for iPhone that supposedly lets you unlock your Mac by simply knocking on your iPhone. It achieves this through using low power Bluetooth LE. Even if the Knock app isn't open, it should still work as long as Knock is running in the background. Or not.
To get up and running with Knock, you purchase the iPhone app and then download the Knock for Mac app via the Knock site (link below). Theoretically, you should just have to pair the two together for Knock to work. I tried this with two different compatible Macs, an iMac and a MacBook Air which were both running OS X Mavericks. On the iMac, just just flat out didn't work. On the MacBook Air, it worked once before my MacBook Air went to sleep and wasn't able to wake back up. I had to hard restart it to get it to turn back on.
I personally wasn't impressed with Knock. Not only for the reason that it didn't work well, but simply because I don't think typing in a desktop password on a computer is that daunting of a task. The app is bound to get killed in the background or take a few seconds to connect anyways. In the time spent fiddling with it, I could already have my Mac unlocked.
Either way, if you happen to try it, let me know if you were any more successful in getting it to work.
More info: Knock