A lot of today's computer usage involves moving between hypertextual links. Unfortunately mostly used method is mouse which puts stress on wrist. There are some alternatives, like providing keyboard shortcuts or combinations to select the wanted element but it's not optimal.
One could imagine instead a system where eye movement was tracked. This could be done for example by using lenses that are put onto eyes (similar to those used for enhancing eye sight). The problem is how a signal can be sent without annoying the user or other problems. An infrared solution is possible, perhaps.
What I find interesting and important for a cheap solution, is that most traversing across the screen is not needed to be very accurate. One can imagine that programs would expose their elements for the application-agnostic tracking softare, and it would pattern match to nearest element, depending on what mode user had put on by keyboards. Of course, this can be automatically context-sensitive when switching between programs. For example, when browsing, the important elements are html's reference and input elements.
For the selection action, a keyboard press, or a wink of an eyelid is sufficient.