If replacing the brake pads and rotors does not solve the problem, you may have an issue with one or both of your calipers. Often calipers can sieze partially or completely. This will make the brake distribution uneven between the front wheels and can lead to shaking or pulsing when applying the brakes.
Tune up
Engine miss at idle. Can be caused by vacuum leak.
http://www.2carpros.com/topics/shakes1.htm
It could be a shifted belt in one of the tires.
bent wheels or a broken cord in a tire or a deformity in a tire.
Perhaps front rotors are "out of round" and need to be either re-surfaced or replaced
Perhaps spark plug wire got disconnected in error?
If it varies with driving speed, look for an out-of-round tire. If it vibrates only when braking, you probably have a warped brake rotor.
It's PROBABLY a wheel balance problem. It just happens that that's the resonance frequency of the suspension.
Sounds like an engine misfire. Can be bad spark plug, plug wire, fuel injector, or bad coil on plug.
This is typically caused by severe wheel alignment problems. Bad suspension components (e.g. shock absorbers) can cause this also.
A dance where it starts out with one person dancing, then like a flash mob everybody starts dancing to the song Harlem Shake.