Depth perception - detect how far into a scene a user is looking
If you know the gaze direction angles of each eye (as well as head position and orientation), then the corresponding rays intersect roughly at the fixation point in 3D. 
This straightforward approach should work in theory, but in practice it is still quite challenging due to both technical and physiological factors.

Pfeiffer, Latoschik and Wachsmuth summarize quite accessibly the state of the art in 3D eye-fixation tracking as in 2008. Here, the intuitive approach I just described was still outperformed by another algorithm.

As the following link suggests, however, the problems with geometrical approaches may have been resolved since 2008. This commercial application appears to use the intersection-point method, though I doubt that the applied algorithms are published.