Am I fascinated with consumer technology or fascinated by it? What about a book, an object, or a philosophical idea?
With: If the object of the fascination is something that can be held, handled, or manipulated (Rubik's cube for example), then someone can be fascinated "with" it.
I was fascinated with his car.
By: If - for example - someone is reading an interesting article, they may be fascinated "by" it.
I was fascinated by her voice.