Mac: FlyingWindows forces you to focus on a single application at at time by either hiding or minimising everything else. You choose the method, and it'll pull any apps that aren't in focus into the background so you're not distracted by their presence.
If you don't have fullscreen mode in your version of OS X or don't want the restrictions it imposes, FlyingWindows offers the same single-app focus without them. You can even set a delay if you need certain windows on the screen for a number of seconds before hiding them.
If you need to focus but don't like OS X's fullscreen mode, FlyingWindows is a good, free alternative. Check it out on the Mac App Store.
Flying Windows (Free) [Mac App Store]