System Preferences -> Appearance -> Double-click a window's title bar to minimize.
BTW, just discovered a new setting related to this. System Preferences -> Dock -> Minimize windows into application icon. Doesn't give you the visual feedback of how many windows are minimized, but works cool with the new Expose mode, since windows that are minimized will then pop up into the Expose view, then hide again once you select the one you want.
jW