Easily create new Address Book Smart Groups: "To quickly create a Smart Group in Address Book without using the menu option, or if you don't use keyboard shortcuts often:
Hold down the Option key, and the plus sign in the Groups column (lower left button) changes to a Smart Group icon.
[robg adds: This behavior carries over from iTunes and iPhoto, both of which work in the same way. I thought we'd documented the Address Book behavior here before, but I couldn't find it in the system.]