If it happens only when you are braking, it may be a bad rotor, or it may be that the problem is limited to or more pronounced on the front of the car.
Posible sources of vibration while driving could be an improperly installed wheel, loose lug bolts, out of balance or out of round tire, wheel bearing, CV joint, bent or off-balance half-shaft, strut/spring problem, steering joint problem, etc.
That's why I'd like to know more about the symptoms. /images/graemlins/smile.gif Then we can help point to a reasonable "narrowing down" process.
Let's say that it happens in straight driving, at a given speed. Can you increase the symptoms by accelerating (shifting the weight to the rear axle) or by decelerating (shifting the weight to the front axle)? If so, rotate the tires front-to-back, and see if the symptoms follow the tires (pointing to a tire/wheel problem), or stay the same (pointing to suspension/hub/axle problem).
Keith