If you run Windows, use a mouse with a scroll wheel, and routinely have multiple windows open, then you need to check out Katmouse.
Katmouse will allow you to scroll a window under the mouse pointer regardless of whether that window has keyboard focus. If you’ve never had functionality like this before, you don’t know what you’re missing, and you must try it out.
Katmouse also adds functionality to the wheel button itself — easier to test out than to explain.
Additionally, Katmouse is nicely configurable, can easily be turned on/off from the system tray, and is of modest overall size.
Oh, and did i mention that it’s free? (There is a Donate button. Once you try out and appreciate the program, go back and click on it.)