I don't know if this belongs on the list or not, but the space bar in the browser works almost like page down. It does however jump too far down, so I don't know if it's really that usable.

Edit: Another one I discovered today is when you use the arrow keys to scroll up and down in a document: When you want arrow down, you need to press Fn (blue button) and arrow down.

If you let go of the blue button before you let go of arrow up or down, it will continue scrolling until you press for instance arrow left. An easy way to scroll all the way to the top or bottom "hands free". Same thing can be done, of course, by just keeping arrow pressed until you are where you want in the document.