If you are using Windows 7, you are most likely to be wondering where is the hell is the good "Show Desktop" shortcut in Taskbar which used to allow users to minimize all running program windows in a single click.
Actually "Show Desktop" shortcut has been moved to end of Windows 7 Taskbar in form of a small rectangle. You can read more about it here:
Has Microsoft Removed Show Desktop Shortcut in Windows 7?
But if you still want to get the old "Show Desktop" shortcut back in Windows 7, then here is how to do it:
1. Create a new folder anywhere you want. For example you create a folder in E: drive. You can give it any name.
2. Now paste following code in Notepad and save the file with name "Show Desktop.scf" (including quotes) and the location should be the new folder which you created in 1st step:
[Shell]
Command=2
IconFile=explorer.exe,3
[Taskbar]
Command=ToggleDesktop
NOTE: You can download a ready-made "Show Desktop.scf" file from here:
Download Show Desktop.scf File
3. Now right-click on taskbar and select "Toolbars -> New Toolbar...".

It'll open Browse dialog box. Select the folder which you created in 1st step and click on "Select folder" button.
4. It'll immediately add that folder to taskbar.
5. Now right-click on taskbar and select "Lock the taskbar" to unlock it.

6. You'll see a placeholder to drag the new toolbar.
7. Right-click on the toolbar placeholder and uncheck "Show Title" and "Show Text" options.

8. If you want to increase the icon size, right-click on the toolbar placeholder and enable "View -> Large Icons" option.

9. Drag-n-drop this toolbar to the left of taskbar.
10. Now lock the taskbar again and you have the good ol' "Show Desktop" shortcut back in Windows 7.

NOTE: If you want to add "Show Desktop" shortcut in Quick Launch toolbar, simply download or create "Show Desktop" shortcut using Step 2 and then put it into Quick launch toolbar as mentioned in following tutorial:
How to Get the Good Old “Quick Launch” Toolbar Back in Windows 7?