Windows only: Distraction-eliminating application CinemaDrape covers your desktop with a black screen except for the application you're currently working in, so you can eliminate background distractions and help yourself focus on getting something done.
The application is provided in both the installable and portable varieties, but the installed version is easier to use — you can simply use the Ctrl+F12 shortcut key combination to launch the dark screen effect for the current window under the mouse. If you were using the portable variety instead of the installed version, your best bet would be to setup a shortcut and assign the shortcut key manually.
Once you've switched into distraction-free mode, you can easily resize the dark screen area, use the Esc key to close out of the dark screen, or you can also right-click anywhere in the dark area to access the options menu, where you can setup pre-defined areas or change the other options like opacity and colours. CinemaDrape is a free download for Windows users only.