Leopard has a new feature that allows you to scroll unselected windows using the scroll wheel of your mouse or the trackpad. As far as features go, it doesn't sound like much. But if you have Leopard, try it. You'll be surprised. I have a large dual-screen desktop and I am using this all the time now. It really cuts down on the amount of clicking I need to do. And it also cuts down on accidental typing in the wrong window.

One thing I've noticed though is that you can not scroll unselected Firefox windows using this trick. It does work for Safari though -- along with all the other applications I've tried it on.