The Woman Who Gave the Macintosh a Smile | The New Yorker:
The command icon, still right there to the left of your space bar, was based on a Swedish campground sign meaning “interesting feature,” pulled from a book of historical symbols. Kare looked to cross-stitch, to mosaics, to hobo signs for inspiration when she got stuck. “Some icons, like the piece of paper, are no problem; but others defy the visual, like ‘Undo.’ ”
[It doesn’t get any better than the original work done for the Mac. This stuff is so good that despite people not having used a computer before, a GUI, etc. so many people recognized what they should do with it. Astonishing work.]
Source: Daring Fireball