This one surprisingly went undiscovered (by me) until just now. I like to use the keyboard and trackpad on my PowerBook a lot, and I am always looking for ways to work more efficiently. I frequently switch applications using the Command-Tab (or Command-Shift-Tab) keyboard combo. Sometimes after Command Tabbing, I'll use the trackpad and pointer to move the mouse to an application that is more than a few positions away from the current app. This involves (as it turns out) an unneccessary movement of the mouse. While command-tabbing, you can two finger scroll, and this will run through the open applications and avoids having to move precisely to the individual application. Pretty slick, pretty simple.
[robg adds: Similarly, the scroll ball on the Mighty Mouse also works for switching applications in the Command-Tab interface. I thought we'd at least mentioned the Mighty Mouse method here before, but it doesn't look like it.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070707202800137