Wizmouse is a free utility that makes your mouse scrollwheel work wherever you point your cursor.
Normally, you would have to click in a window to bring it into focus or make it into the active window and only then can you scroll.
But with Wizmouse installed, you can be working a particular window and without leaving focus, move your cursor to another window and scroll through its contents.
It might appear to be a small thing but it can be a great time saver besides saving you the annoyance of moving back and forth between the windows.
Optionally it can also add wheel-scrolling capabilities to programs that don’t support it.