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 = 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 + # = Quicklaunch
18. Windows Key + = = Magnifier

From Windows Explorer

19. Alt + Up = Go up one level
20. Alt + Left/ Right = Back/ Forward

[http://blog.taragana.com/index.php/archive/ 20-windows-7-shortcuts-you-must-know/]