Props for linking to OpenGazer and Dasher. AEGIS is absolutely a terrific project, and I've been inspired by the work that Emli-Mari Nel and others are doing on exactly this problem.

Something to keep in mind when you're thinking about eye tracking for accessibility is how much custom hardware can greatly enhance the experience. Even if you're just mounting a webcam on a head bracket and removing the IR filter, some inexpensive work can really improve the user interface. (Of course, mounting a camera on your face isn't necessarily conducive to unbiased usability testing!)

One of my greatest hopes, personally, is that our approach to webcam-based eye tracking will yield algorithms and methods that we can give back to the community. Eye tracking has its shortcomings, to be sure, but the potential benefit is too great to be ignored.