Click app in dock to show open instances
While generally most applications allow only for one instance of an application to be running, I believe that you mean the multiple windows open for an application.
By default OS X does not show icon or window previews on the Dock. However, there are two options for allowing to see the windows:
- Mission Control (built-in)
Mission Control gives you the overview on the entire screen with all open application windows grouped together, but there is also an integrated function, called "Application Windows", which just shows you all the windows open for the currently active (foreground application). You can assign keyboard shortcuts and and trackpad gestures inside System Preferences. Additionally, if you prefer to use the mouse for this feature, you can drag the Mission Control application into the Dock for you to click on. - 3rd party application (e.g. HyperDock)
There are some 3rd party applications, which mimic the Aero window preview of Windows 7. The one I used to have was called HyperDock, which looks almost like the real thing, but there are others as well.
Apple provides the Mission Control visualization and it's not designed to work in even the slightest manner similar to the windows task bar.
If you prefer the task bar, you will want to look for third party window manager and task bar software providers - there are always several good solid ones and some up and coming ones.
Also - check out fluid app - it's really nice for web apps to make them feel more like a real app as opposed to just another tab or window in your browser. It's not for everyone, but those that like it - absolutely love it in general.
- http://fluidapp.com