From Desktop

1. Windows Key + Tab : Aero [press Tab to cycle between Windows]
2. Windows Key + E : Windows Explorer is launched.
3. Windows Key + R : Run Command is launched.
4. Windows Key + F : Search (which is there in previous Windows versions too)
5. Windows Key + X : Mobility Center
6. Windows Key + L : Lock Computer (It is there from the earlier versions as well)
7. Windows Key + U : Launches Ease of Access
8. Windows Key + P : Projector
9. Windows Key + T : Cycle Super Taskbar Items
10. Windows Key + S : OneNote Screen Clipping Tool [requires OneNote]
11. Windows Key + M : Minimize All Windows
12. Windows Key + D : Show/Hide Desktop
13. Windows Key + Up : Maximize Current Window
14. Windows Key + Down : Restore Down / Minimize Current Windows
15. Windows Key + Left : Tile Current Window to the Left
16. Windows Key + Right : Tile Current Windows to the Right
[Continue pressing the Left and Right keys to rotate the window as well]
17. 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.
18. Windows Key + = : Launches the Magnifier
19. Windows Key + Plus : Zoom in
20. Windows Key + Minus : Zooms out
21. Windows Key + Space : Peek at the desktop

From Windows Explorer
22. Alt + Up : Go up one level
23. Alt + Left/ Right : Back/ Forward
24. Alt + P : Show/hide Preview Pane
Taskbar modifiers
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 afresh.