Some of my most useful tips (to me anyway) come from mistyping or being clumsy on the keyboard. The other day there was something leaning on my shift key while I was using the roller on my mouse and lo and behold, I scrolled sideways instead of up and down. Cool! I'm not sure if that's mouse-specific (mine's a wireless Logitech), but I thought I'd pass it on.
Hmm. It looks like the left-right mouse scrolling is at least application dependent. In Firefox, Shift makes the mouse scroll wheel go left and right (instead of the default up and down), but in InDesign CS4, the Shift key super-charges the scrolling wheel so you scroll faster, but still up and down. The Option key plus mouse scroll zooms in and out, and the Command key finally makes the mouse wheel scroll left and right. I can't find a place where you can change these mappings.