Backdrop!

LeDimmer is a small Windows application (<200KB) that sits in the system tray and simply dims the screen around the foreground window.

Clutter Cloak is a tiny anti-distraction program for Windows XP.It is activated by a configurable hotkey, then obscures everything on the screen except the child window immediately under the mouse pointer. (By default the hotkey is Ctrl-F11.)

Windows only: Instantly blank out your desktop and all of your running apps with Dropcloth, a Windows copy of previously mentioned Mac-only app Backdrop.