Piles of documents. Some interesting speculation on Mac Rumors about Apple integrating a finder feature called Piles that creates a finder metaphor based on the physical act of viewing/sifting through a pile of documents on a desktop in meatspace. Here's a description from an earlier Tog article.
Apple holds a patent on this one. Developed by Gitta Salomon and her team close to a decade ago, a pile is a loose grouping of documents. Its visual representation is an overlay of all the documents within the pile, one on top of the other, rotated to varying degrees. In other words, a pile on the desktop looked just like a pile on your real desktop. To view the documents within the pile, you clicked on the top of the pile and drew the mouse up the screen. As you did so, one document after another would appear as a thumbnail next to the pile. When you found the one you were looking for, you would release the mouse and the current document would open. Piles, unlike today's folders, gave you a lot of hints as to their contents. You could judge the number of documents in the pile by its height. You could judge its composition very rapidly by pulling through it.