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.
Source: Yahoo Answers.
(i) "by" names the agent responsible for the action.
By is commonly the preposition whose object is the agent of the passive verb:
I was fascinated by her voice: This is the passive form of Her voice fascinated me.
(ii) "with" names the instrument that was used for/caused the action.
I was fascinated with his car. Here, fascinated is an adjective and "with his car" modifies fascinated.