If you’re using Apple’s Mighty Mouse—or possibly any third-party mouse with a scroll ball—there are a few spots where you can use its scroll ball that you may not expect. There are the relatively well known spots, of course—for doing things like zooming the screen (hold Control and move the scroll ball), switching applications (press Command-Tab, then move the scroll ball), and setting the volume in iTunes or QuickTime Player (hover over the volume slider, then move the scroll ball)
Here are a couple spots you may not have thought to try it, though. In OS X 10.5, if you use Spaces, press F8 (to enter Spaces’ overview mode), and then move your mouse’s scroll wheel. As you do, you’ll see each available Space highlight in a logical sequence—move the scroll button up and down to select between vertically-aligned spaces, and move it left and right for the horizontally-aligned spaces. To select a highlighted Space, do not click the mouse button, as that will instead activate whichever space is under the mouse cursor. Press Return, though, and the highlighted Space will become active.
You can also do this same thing in Exposé’s All Windows (F9 by default) mode. Press F9, then move the scroll bar around, and you’ll see that each Window highlights in a logical if not entirely predictable manner. Up and down movement of the scroll bar generally selects vertically-aligned windows, and sideways motions do the same for horizontally-aligned windows. Again, to select a highlighted window, press Return—don’t click the mouse, as your pointer may be anywhere on the screen. (This particular tip may only work in 10.5; my last 10.4 machine left the premises last week, and I haven’t had a chance to set up a new test partition yet.)
There are probably many other spots where the scroll ball works that aren’t all that common. If you know of any, post them in the forums.