Spring-loaded folders

From Applepedia

Spring-loaded folders are a feature of the MacOS Finder, since MacOS 8.

Folders

When you drag a file onto a folder, hover over it for a few seconds, and it'll pop open. You can keep doing this until you reach your destination. Now when you release the mouse, it'll close all the intermediary folders. To speed things up, you can also press the spacebar to immmediately open the folder, rather than waiting for it to open on its own accord. If you accidentally went to the wrong folder, simply move your mouse back into the area of the previous folder, and the incorrect window will disappear.

Magnifying glass (OS 8 & 9 only)

The Finder in MacOS 8 and 9 supports a "click and a half". Double-click, but on the second click don't release your mouse button. The cursor will turn into a magifying glass, which will let you open folders by hovering over them, as if you were dragging a file.