I've never noticed this before and I think it came with the Spring Loaded folders. Open a finder window on the desktop, and just for fun make it decently wide (say - half the screen width) and select column view.
Now, drag it off to the side of the screen so only a small bit of the side is visible. I chose the left side, so that the right-side of the window is there, just wide enough to see the toggle vide gadget.
Now - here's the fun part! Make a folder or take an icon from the desktop (not a drive icon!) and drag it to the window, but don't release it. Depending on your Spring Loaded folders option, your window should slide open for you! In column view, you can dig around and slide the window contents right and left (no matter how wide the window is), and once you slide out, the window slides back to the side of the screen!
I keep my Downloads directory on one side, and my Sites directory on the other. It's great fun ... although the schoolgirl-giggling doesn't go over well with the Windows users ;-).
[Editor's note: Very interesting and potentially useful behavior ... and a cool demo, if nothing else!]
Mac OS X Hints
http://hints.macworld.com/article.php?story=2002090207243137