Scroll Reverser is a free app for OS X that reverses the direction of scrolling. You can use it with OS X 10.5 and OS X 10.6 (see Related Links for older version) to make your scrolling match the 'natural scrolling' in OS X 10.7.
You can also use it with OS X 10.7 to customise scrolling behaviour beyond what is possible in system preferences, for example to reverse the trackpad but not the mouse.
Scroll Reverser is small, simple and efficient and works with all mice and trackpads with a scroll wheel or scroll gesture support, including MacBook trackpads, Magic Trackpad, Magic Mouse, and Mighty Mouse.
Note: While the software is classified as free, it is actually donationware. Please consider making a donation to help support development.