WizMouse is a lightweight program that lets your mouse wheel scroll the window under the mouse pointer even if the window doesn't have input focus. This means you no longer have to click on a window to activate it before you can scroll it with your mouse wheel.

The program can also optionally enable the mouse wheel in applications that don't support mouse by converting mouse wheel messages to scroll bar messages.

Other options include "reverse mouse scrolling" (like "natural scrolling" on Mac OS X Lion), and bringing the scrolling window to the foreground.