Whenever I'm whiling away at my computer, I like to keep my hands firmly planted at my home row whenever possible. That means I get bummed whenever I have to move to my mouse scroll wheel, or to the arrow keys or page up/down keys. That's why I'm gaga for using the space bar as page up/page down in Firefox.
You may have already noticed that an inadvertent space bar press pages down the web page you're currently viewing, which by itself is all right useful, but couple that with Shift-space bar and you've got the tools to page back up, letting you navigate around a page entirely from the home row. For non-Firefoxers, the same is true for Safari