The Dock in Mac OS X has the ability to hold folders and stacks.
One of the cool features of the Mac OS X Dock is the ability to create stacks and folders on it. You can group applications, documents, or other forms of data that are easy to reach on the Dock. As we all know, Mac OS X Lion was released with the intent of adding numerous forms of multi-touch gestures to Mac OS X. With three-finger dragging, two-finger scrolling, and four-finger paging between applications, Mac OS X Lion has made it possible to eliminate any kind of mouse altogether. You don't even need to click with the multi-touch pad because you can just tap to click.
Another feature that you may not have known about is that using a hidden terminal command, you can enable multi-touch gestures to open and close folders or stacks on the Mac OS X Dock. Alternatively, if you use a traditional point and click mouse, using the scroll wheel will open the folder as well. To do this, hover your pointer over the icon that is either a stack or a folder, and use either the mouse's scroll wheel to open it, or use the Mac's Magic Mouse or Multi-touch pad to scroll to open the stack or folder. It's that simple.
Enabling this feature is done through a terminal command which switched a Boolean variable value to true and then restarts the Mac OS X Dock. The command you need to enter is:
defaults write com.apple.dock scroll-to-open -bool TRUE;killall Dock
defaults delete com.apple.dock scroll-to-open;killall Dock
Do you like this feature? Tell us why in the comments!
Sources: OSX Daily