Lion OS X changes the direction of travel by default as of IOS; we slide the finger up to go down, and down to go up. This decision failed at first, but you soon ... at least on a trackpad. With a mouse, the gesture is less clear, and if Apple has provided two options, one for the trackpad and one for the mouse, a bug that persists in OS X 10.7.1 to prevent two different choices. If you set the trackpad scrolling natural mouse will also adopt this behavior, whatever you do.
Scroll Reverser (200 KB) originally allowed activating the scroll Natural Snow Leopard, to facilitate the transition. The small free utility functions nevertheless won since its first release and is now very useful also in Leo, especially if you use a trackpad sometimes, sometimes a mouse. Scroll Reverser makes first (un) natural temporarily activate the scrolling. Better still, it is able to do what Apple does not: enable it for certain types of devices only. This will be used with a trackpad, but not with a mouse, for example.
Another option offered by Scroll Reverse: you can activate the natural scroll vertically but not horizontally. The current version does not, however, the intersection of the two options (natural horizontal scrolling trackpad for example). These options also work for the two-finger scrolling, not three or more fingers.
