There are all kinds of great gestures for MacBooks, Magic Trackpads and the Magic Mouse. Over on iMore, they show off how to add one more that reveals what windows are open in an app.
Once you enable the gesture with a Terminal command, you can do an upward two-finger swipe over an app icon in the dock to reveal all the windows open in that app. It will also reveal the contents of any folder. To turn it on, open up Terminal (Applications > Utilities) and type in:
defaults write com.apple.dock scroll-to-open -bool TRUE; killall Dock
Now, when you swipe over any icon in your dock you'll see all the open windows associated with that app. Swipe fown to return to normal.