Minimize any program in to the system tray.
For example for Minesweeper, Freecell or a certain webbrowser. You can define by program ("Minesweeper.exe") or window title ("Facebook") which program gets minimized by the boss key.
More inconspicuous that a key.
It's enough (if you activate that) to more the mouse cursor quickly into the screen corner. Immediately the configured programs are minimized into the tray. You don't even have to push a key (even though that would be possible, too). The selected programs are even minimized if they're in the background.
Simply click on the tray icon and the respective program is back again.