Windows Key + Tab : Aero [press Tab to cycle between Windows]
Windows Key + E : Windows Explorer is launched.
Windows Key + R : Run Command is launched.
Windows Key + F : Search (which is there in previous Windows versions too)
Windows Key + X : Mobility Center
Windows Key + L : Lock Computer (It is there from the earlier versions as well)
Windows Key + U : Launches Ease of Access
Windows Key + P : Projector
Windows Key + T : Cycle Super Taskbar Items
Windows Key + S : OneNote Screen Clipping Tool [requires OneNote]
Windows Key + M : Minimize All Windows
Windows Key + D : Show/Hide Desktop
Windows Key + Up : Maximize Current Window
Windows Key + Down : Restore Down / Minimize Current Windows
Windows Key + Left : Tile Current Window to the Left
Windows Key + Right : Tile Current Windows to the Right
[Continue pressing the Left and Right keys to rotate the window as well]
Windows Key + # (# is any number) : Quickly Launch a new instance of the application in the Nth slot on the taskbar. Example Win+1 launches first pinned app, Win+2 launches second, etc.
Windows Key + = : Launches the Magnifier
Windows Key + Plus : Zoom in
Windows Key + Minus : Zooms out
Windows Key + Space : Peek at the desktop
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From Windows Explorer

25. Shift + Click on icon : Open a new instance
26. Middle click on icon : Open a new instance
27. Ctrl + Shift + Click on icon : Open a new instance with Admin privileges
28. Shift + Right-click on icon : Show window menu (Restore / Minimize / Move / etc). Note: Normally you can just right-click on the window thumbnail to get this menu.
29. Shift + Right-click on grouped icon : Menu with Restore All / Minimize All / Close All, etc.
30. Ctrl + Click on grouped icon : Cycle between the windows (or tabs) in the group

Though some of them are there in previous versions of Windows, Windows 7 has incorporated many of them and have started many as fresh.