Eyeboard

A 17 year old in Honduras has designed a low cost system (<$300) for eye tracking built into a pair of glasses.  It is accurate enough that disabled people who don't have use of their limbs can use it to spell out words on a computer just by looking at the letters.

Systems like this exist, but usually are much more expensive.  He is releasing the code as open source in hopes of speeding up development.  You can read more about the method he used, called Electroculogram, or see his rather impressive documentation.