A Mental Head's-Up-Display. This is something I wish I had. Imagine being able to instantly annotate something in your mind and have it appear in your field of vision, available for your perusal at any time, and static. This would come in very handy when organizing your thoughts, trying to remember a phone number or address, coming up with Things To Do Lists, and so on.

Actually, we already have something sort of like a mental HUD; we can visualize things in our mind quite easily. However, it's not static, and if you visualize a phone number and then become distracted doing something else, you are likely to forget it.

A good example of a mental HUD is the one that the Terminator has in the movies of the same name.