My seersucker suit
It's hot enough that it came out of the closet for today's speech.
It totally transformed the way people treated me. Doormen, people on the subway... in an increasingly casual age, I was sort of stunned by how easily a $99 suit changed the reaction people had.
When everyone (men, anyway) wore the same thing, it was pretty difficult to make an impact with your clothes. Today, it's a conscious choice and it matters whether you want it to or not.
PS six years ago, at Yahoo, Jerry Yang caught me wearing a suit while he was giving a tour to a bunch of hotshot visitors. He sent me home.