I have long been a fan of electroencephalogram's (EEG) a.k.a. brainwaves. An EEG monitors cranial voltage fluctuations through the skin. These voltage fluctuations are caused at least partially by the activity of neurons in the cerebral cortex (the wrinkled outer layer of the brain). Despite the mind reading power of "brain waves" in countless sci-fi incarnations there is really very little information there to be gleaned. If I may make a comparison I would say that looking at brain waves is more or less equivalent to watching the clock rate of a computer. Our brains are massively parallel computing devices and different regions of our brain ramp up their activity for different tasks. Brain waves somehow correspond to large scale activity of the cortex and while this information can certainly be valuable it ultimately carries far too little information trying to read minds using brain waves would be like trying to index the content of the internet using only character frequencies.
That is not to say that brain waves can't be used to garner some interesting information and do interesting tasks. Brainwaves can be used to determine level of focus and people can be trained to control their brainwaves to a certain extent. This has been used for some interesting devices (the emotiv device is probably the closest to having any sort of real usefulness. here is a nifty if a bit long video which includes a demonstration of the device done with none other than AI legend Marvin Minsky)
But ultimately such devices are far too limited in their information collecting ability to carry out tasks of real utility like say typing at a reasonable rate with low error. The first most obvious way to get better information is to have direct brain implants but although this might turn out to be the best way to go many decades from now since such an interface has the bonus that we can send information in as well as extract information. Such direct neural interfaces are being developed now for control of artificial prostheses complete with tactile feedback thanks to DARPA. But, even after we solve all the problems of direct neural interfaces (like decreasing sensitivity due to the slow build up of scar tissue) not everyone will want to have brain surgery so that they can check facebook by thinking.
The most elegant to get at the information locked up in our skulls depends on the use of SQUIDs. A SQUID is a Superconducting Quantum Interference Device and it enables unimaginably sensitive measurements of magnetic fields. Squids have already been used to measure magnetic fields caused by neuronal activity. In order to even detect such fields one needs femto tesla range sensitivity. It is wonderful that SQUIDs can deliver this sort of sensitivity but highly unfortunate that SQUIDs currently require cryogenic temperatures and low fields in order to operate. Let us imagine for a moment however a world in which room temperature super conductivity had been discovered and furthermore this room temperature superconducting material can be printed on silicon in the manner of current integrated circuits. In short what if one could print SQUIDs on silicon just like we can make CCD detectors for cameras. Thousands or millions of magnetic field detectors each powerful enough to detect the firing of single neurons all wrapped around our brains picking up the signal sent out from each individual neuron firing.
Admittedly such a technology is at the very least 20 years distant to be naively optimistic or the technology may even be fundamentally impossible since there is no evidence to support that room temperature superconductivity is achievable. But here at least is a scientifically plausible route to totally non-invasive and incredibly powerful brain monitoring device. This is my prediction for the "killer app" of a high temperature super conductor if one is ever found.