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Scroll Reverser for Mac OS X
Scroll Reverser is a free app for Mac OS X that reverses the direction of scrolling. You can use it with OS X Tiger, Leopard and Snow Leopard to make your scrolling match the 'natural scrolling' in Lion and Mountain Lion.
On OS X Lion and above, use it to customise scrolling behaviour beyond what is possible in system preferences, for example to reverse the mouse but not the trackpad. It is also useful for getting some Wacom tablets to scroll properly.