MouseWinX is a Windows tray application that lets you quickly toggle X-mouse
window activation, where the window focus follows position of the mouse cursor
without having to click anything.  I wrote this back in college when I was
using Magic VLSI for one of my classes, and I've also found it helpful for
navigating GIMP.  Both are applications with multiple windows to interact
with, and hot-tracking the mouse makes them a bit easier to use.  But the rest
of the time I don't like having the window focus jump around, so MouseWinX
gives a simple icon in the tray which toggles the setting when clicked.

That's all there is to using it -- just run the app, and toggle mouse-tracking
by clicking the icon in the system tray.  A right-click on the icon will let
you bring up a Settings window where you can fully tweak the behavior.

You probably also want to tell Windows not to hide the icon.  On Windows XP,
this is found by right-clicking the taskbar, then go to Properties -> Taskbar
-> Customize, find the MouseWinX icon and change to "Always Show".  On Windows
Vista and Windows 7, click the arrow next to the tray and go to Customize, and
then for the MouseWinX icon choose "Show icon and notifications".