Mac: Most Mac users know how to hide an application, but when it's not the application you're working in that needs to go away, Cmd+Opt+H will hide all of the other apps running so you can focus on what you're doing and get some work done.
This shortcut is a permutation on the more commonly known Cmd+H, which hides the front-most application and all of its windows. Unfortunately, there's no native shortcut to show all running apps again if you've accidentally hidden everything but the one you're working in, so to restore them click the Apple menu and select "Show All".
Shortcut of the Day highlights a great shortcut on various platforms from the desktop to mobile devices to the web. You can find more shortcuts here.