I've been lucky enough to play around with a NextMind dev kit. These awesome devices use electrodes that are pressed to the back of the head to.... well... read your mind.
Once calibrated, the NextMind devices can utilise your 'focus' on a screen, to eerily predict what part of a screen you are looking at. The best way i can describe what's happening is to explain it as eye tracking software - except no cameras. Rather than looking at your eyes to see what you are focusing on, NextMind looks into your mind through the back of your head to achieve this.
The result is honestly scary. With a good calibration score the accuracy is very very good. There is, understandably, a learning curve of how to focus your attention for the device, and also a slight delay in the reaction of the device.
A interesting benefit of the PIN demo show above is that a 'discreet' mode can be activated, so even onlookers won't know what is being input, it's a secret between your brain, focus, the NextMind, and your computer.
And I know what you're thinking. Maybe it knows what you're SUPPOSED to be thinking. especially in terms of a game, or a PIN number, it's reasonable to expect the system knows the expected result and could make a better guess of what you input. This falls down if you purposefully do incorrect inputs. I can confirm, even when purposefully selecting incorrect options, the NextMind matches what you select perfectly.
How exactly it works we don't know, but its certainly good, and its certainly unnerving...