When Leopard was released one of the coolest features was Stacks. Stacks were basically just folders sitting in your Dock that when clicked on would fan out to display all of the contents of that folder. Stacks Folders however is a new feature for Snow Leopard that will allow you to navigate through your computers files system from within those stacks.
There is no indications as to whether or not this will be a final feature of Snow Leopard but if it is I’m sure it will end up much more polished than its current incarnation because honestly, it looks a little messy.
Click the link below for a video demonstration of Stacks Folders: