In applications that support scroll wheels (all Cocoa apps, some Carbon apps), when you pull down a menu, you can use your mouse's scroll wheel (if you have one) to scroll through the menu's choices. Also, for the duration you're in the menu, your scroll wheel will act like a button when you press down on it.

[Editor's note: Cool trick! I found that the button behavior was flakey (it's ignored in Mozilla, for example), but the scroll wheel worked everywhere I tried it in supported apps!]