How do I minimize all the windows using a keyboard shortcut?

For 10.10, 13.10 and 17.10

Super+D is the default keyboard shortcut for minimizing all windows in Ubuntu 10.10.
You can change it to Super+M like it is in Windows using the Keyboard Shortcuts utility:
System ➜ Preferences ➜ Keyboard Shortcuts

Note: The keyboard shortcut Super+M is used by both the Messaging Menu (see this question for a solution) and the Negative Compiz plugin (You can disable or change its keyboard shortcuts by using compizconfig-settings-manager).

12.04 and later

The keyboard shortcut to minimize all windows is Ctrl+Super+D in Ubuntu 12.04 and later.

Simply press and hold ,bkd>Super key(Which is generally the key with Windows logo on it) you will get list of all the shortcuts. in my Ubuntu 12.04 minimizing all windows in Ctrl+Super+d