Stacks for Unity Launcher

Solution 1:

Give a try to Cairo Dock, whose interface is easy to set up as you wish and its gadgets includes stacking with several fancy features.

Screenshot from my Desktop:

More screenshots from the net:

Parabolic style sub-dock:

Dock over dock style

Slide style sub-dock

Rainbow style sub-dock

Good luck!

Sorry, I forgot to mention the Panel Mode, which is cool, an illustration is placed here:

Solution 2:

Check out Drawers.

You can put just about anything in a drawer by drag and drop. You can even nest drawers inside one another. See below for a screenshot. It's been submitted to the software center but is awaiting review.

In the meantime, to install drawers run the following in a terminal:

sudo add-apt-repository ppa:ian-berke/ppa-drawers
sudo apt-get update
sudo apt-get install drawers

Then create your first drawer by opening the dash and typing Drawers.

Sources : https://askubuntu.com/a/168563/82505 and the developers of drawer.