Ruled paper

Each sheet of loose-leaf paper would have at least one edge marked off in centimeters (and slightly thicker to prevent tearing). Since random pieces of paper are frequently the only straight edge in arm's reach, this would increase their usefulness by making them the nearest ruler, too. If done properly, the ruler edge wouldn't be obtrusive, and only noticeable when you needed it.

I imagine high school science students would find this most useful. To keep them on their toes and reduce production costs, why not make it a Golomb Ruler? Although, this doesn't make measuring fractional distances very practical.

(On that note, is there a difference between a Golomb Ruler and a Perfect Ruler?)