Adding a "show desktop" button

OroboROX lets you define a keyboard short-cut for this.

For other window managers, or if you want a clickable button, Andras Mohari suggests this script:

if wmctrl -m | grep -q '"showing the desktop" mode: ON'; then
	exec wmctrl -k off
	exec wmctrl -k on

This requires the wmctrl program and was tested using xfwm4 (should work with any other EWMH-compatible window manager that implements the 'show desktop' feature, at least in theory).

Save the script and set its permissions to make it executable (File -> Permissions). You can now drag it to your panel and click on it to show the desktop. You can also bind a keyboard short-cut to it using the panel's Edit Item menu item.