In the near future you could be interfacing with certain applications simply with your eyes. Students at the Imperial College London have shown that such a device could be built for less than $30.
The idea is not as revolutionary as you may think and uses commonly available hardware. The students used game console cameras capable of tracking 3D objects, such as Microsoft's Kinect. However, instead of tracking an entire body, two cameras were placed on a head-mounted construct "outside of the user's field of vision", and were recalibrated to track pupils. According to the students working on the project, the concept worked well, even if the technology was just demonstrated with a simple Pelota-like game.
A commercial product would need "appropriate" funding and about three more years of development, the researchers said.