Cooked up by the Mitsubishi Pencil Company—which sounds like it would have an awesome factory tour—the cheap $1.60 Uni Promark View uses a thinned water-based pigment ink that flows down a channel inside the highlighter's clear plastic tip. The design provides a window so that you can always see the text you're highlighting, and don't accidentally overshoot the end of a sentence. Technically it's not a problem that's ever really cost someone a passing grade, but that doesn't mean it wasn't worth solving.
[Watch Impress]
Why Did It Take So Long to Make a See-Through Highlighter Tip?