Metric paper
I found this article about a year ago and was entralled for an evening. Slashdot just picked it up, reminding me how much I enjoy metric. This is a particularly amazing detail:
Technical drawing pens follow the same size-ratio principle. The standard sizes differ by a factor sqrt(2): 2.00 mm, 1.40 mm, 1.00 mm, 0.70 mm, 0.50 mm, 0.35 mm, 0.25 mm, 0.18 mm, 0.13 mm. So after drawing with a 0.35 mm pen on A3 paper and reducing it to A4, you can continue with the 0.25 mm pen. (ISO 9175-1)
The obsessive-compulsive in me swoons at that level of order!