There's a keyboard shortcut to show the desktop on most (GNOME-based) Linux systems — Ctrl+Alt+D, or sometimes just Windows+D. If you like having an actual button to drop down, à la Windows, you can have that too.
Linux blog Web Upd8 explains the process for creating a desktop shortcut and loading it somewhere convenient. In the case shown there, it's dragging it onto a Docky dock. If that's not your thing, you can also add that new shortcut into your top panel's app shortcut section, or just keep it in the lower-right corner of your desktop, as detailed by tipster Nitrokart (also known as "Barry knows CPR and took that guy's wallet" — comments are fun, sometimes).
'Show Desktop' Applet For Docky (Gnome-Do) [Web Upd8]