I'm going to go ahead and hijack this thread and throw in "7 inch angle grinder/sander/buffer/polisher."
If I could only choose one tool, this does it all, from grinding my crappy welding, remove 2 inches of bondo, to shaping wood, to buffing cars/ guitars, to polishing headlights. With the right touch, it can work wonders on both extremes of the small sanders listed. In conjunction with the square sander mentioned above, they are quite the duo.
