Hold down the shift key and click and hold the mouse on a blank spot on the page. Then move the mouse up or down past the edge of the window, and the window will scroll in the respective direction. I'm not sure if this works in any other apps, but it's definitely not in all Cocoa apps (it fails in TextEdit, for instance).
[robg adds: In Safari, the shift key somehow disables text selection, which is what you'd otherwise get if you just did a click-and-hold and then moved the mouse up or down...]

