Dock Exposé was a Snow Leopard feature which allowed users to click on an app on the Dock and, holding that click, windows of that app would be exposed. On Mountain Lion, Dock Exposé doesn't seem to be available anymore. Is there a way to get it back?
Thankfully, this feature isn't completely gone. It has just, for some reason, been whisked away into the depths of less-than-obvious multi-touch gestures in OS X Lion and newer. You now have to double-tap an app icon on the Dock with two fingers to activate Dock Exposé.
I'm not sure if this gesture ever existed on OS X Snow Leopard.