When I was a Windows user I used to love the quick window preview when you hovered over a program in the task bar - it was a feature I wish OS X had pre-installed. The great thing about OS X is the great array of apps you can get which can change your experience and that's exactly what HyperDock does.
HyperDock brings those quick window previews from Windows and puts them in your dock. When you hover over an app, it'll show a quick preview of that app in real time. If you have multiple windows open for one app, it'll show all of them and you can choose between them.
It doesn't just stop at window previews, it can also give you quick access to controls. If you have iTunes running, you can quickly pause and skip songs. But then you'll most likely be using an Apple keyboard which already has those controls. It also shows any upcoming iCal events which is pretty useful.