I've argued that "free" and "you" are two words you want to use as much as you can.
But just as a quick post, I think the two most demonstrate-able POWER WORDS ever used in advertising can be found on the back of the shampoo bottle.
The two words have to do with repeat sales.
The words are "RINSE, REPEAT".
Who ever wrote those words should be in the marketers hall of fame.