A while back we've posted about a "show desktop" icon for the Unity launcher. But if you need more space on the Unity launcher or have autohide turned on, you may find it more convenient to access this from the panel so WebUpd8 reader Mark Bokil converted the "show desktop" Unity icon to an Ubuntu indicator.
To use it, firstly install wmctrl:
sudo apt-get install wmctrl
Then, download the archive from here, extract it and double click the "desktop-indicator.py" file.
To add Desktop Indicator to startup, search for "Startup Applications" in Dash (or access it via the session menu - top right icon), click "Add", under "Name" enter "Desktop Indicator" and under "Command" click "Browse" and select "desktop-indicator.py" from where you've extracted the downloaded archive.
Thanks to Mark Bokil for the indicator!