When are you "fascinated with" something, and when are you "fascinated by" it?

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.