It is called Avant Windows Navigator (or simpler, AWN), it can be found on Ubuntu Software Center, and it looks something like this:
The dock is not that simple to use as that one from Mac OS X, but after a few minutes you should already know how it works and be able to configure it without problems. But, this dock is even better than that one found on Mac OS X, because you can add many other widgets on it directly from its menu (like real-time statistics about computer or weather). You can put it on any part of the screen, even on the top, and this is something that you can't do with Mac's default dock.
Get it from the Ubuntu Software Center by searching "AWN".