This is a really simple, lightweight extension that does just one thing: reverse the scrolling direction of the Up/Down and Page Up/Page Down keys on your keyboard when viewing web pages.
Why would you ever want to do that? Well, if you've adopted Apple's new scrolling paradigm in OS X Lion--the so-called "natural scrolling"--you may have noticed that the setting does not affect scrolling with the keyboard. If you've gotten used to moving window content up and down with touch gestures, you might want to maintain that metaphor when you scroll using the keyboard. Install NatKeyScroll, and the Up and Page Up keys move web pages up, while the Down and Page Down keys move them down. Makes sense, doesn't it?