If you need to scroll an application window on a PC then spinning the mouse wheel is probably the simplest route. This only works on the foreground window, though, the one where you're currently working. And so if you want to scroll some other window, you must click it first.
AlwaysMouseWheel is a tiny portable tool which changes all this. Once running, you can switch to and scroll any window just by moving the mouse cursor over it, and spinning the wheel - no extra click required.
While this is plainly a very small enhancement, if you regularly work with lots of open applications then it really can make your life a little easier (which is probably why some Linux distros support the same functionality).
There is also one important configuration option. By default AlwaysMouseWheel performs two steps when you spin the mouse wheel; it switches to the window under the mouse cursor, then scrolls it. But if you check "Just forwarding the Wheel command", the first step is dropped. It just scrolls the window you're pointing at, without bringing it to the foreground first.