I NEVER noticed this tool before--and it's so cool. If you want to extract something from a picture, instead of using the polygonal lasso tool and clicking all those points around the item, you can use the Magic Extractor (found at the bottom of the Images menu) a lot more easily! Scribble over what you want to extract after clicking on the plus tool and then loosely around the edges with the minus tool to indicate what to cut away---and it's like magic. (DUH!) I'm going to have to play a lot more with this.
Of course, everyone else probably already knew this, but I'm so psyched!