https://itunes.apple.com/us/app/1appmode/id979862251?ls=1&mt=12

MacOS X Single Application Mode Revisited

Many moons ago, when MacOS X was introduced in 2000, every MacOS window had a toggle button in the title bar. The small button could be used to put a window in "Single Window Mode". When a user clicked on a icon in the dock for a "Single Window Mode" application, all other active applications would be minimized. It was a way for users to focus on one application window at a time and see more of their desktop.

The option never became popular. The capability does still live on to some degree as full screen mode and also as "single application mode" which can be activated via a terminal command. However the terminal command option only works when the user clicks on the Dock to task switch. Command-tab switching ignores the setting.

1AppMode is different from both those options. It's just a global toggle switch that resides in the status bar menu. Activating it hides all application windows that are not active. It keeps only the current application (and all its windows) visible. Whether you Command-Tab or click on the dock, it tries to keep one application visible at a time. Sometimes it screws up when it comes to things like X11 apps that launch terminal windows, so you have the option to "Turn Off" 1AppMode in its menu if things get whacky.

It also works with the desktop such that when you click the desktop the Finder/desktop becomes active.

For MacOS 10.7 and higher.