WizMouse is a lightweight program that lets your mouse wheel scroll the window under the pointer, even if the window doesn't have input focus. Normal behavior requires clicking on a window to activate it before you can scroll it with your mouse wheel.

The program can optionally enable the mouse wheel in applications that don't support it 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.