Keycard is a new Mac app that pairs to your iPhone, iPad, or iPod touch with Bluetooth, and when it detects that you've stepped away from your Mac (assuming you have your device with you), it locks your Mac. Once you return, or enter in the 4 digit code, your Mac unlocks.
To use Keycard, simply make sure Bluetooth is enabled on your iPhone or iPad, confirm it's connected to Keycard on your Mac, and enter in a four digit password. Keycard also includes options to open at startup and hide from your dock.
The four digit password may seem like an odd feature of the app, since it's all about your proximity to your computer, but this feature is actually very important. Suppose your iPhone's battery dies when you're away from your desk or even worse, it gets stolen. Without the 4 digit pin, you would be locked out of your Mac until you got your iPhone back.
I've tested out Keycard at home and it works great. I love how it unlocks before you're actually at your desk because it doesn't slow you down at all and disrupt your workflow. However, it would be nice to adjust what distance you need to be from your Mac in order for it to lock and unlock as some people may want to shrink that distance to only a few feet.
If you've been known to have your social networks "hacked" by your friends when you step away from your computer, you may not want to pass this one up.